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Volatile Endeavors: 
Negotiating Left Turns Along the SE Hawthorne Bike Corridor 
Aaron Smith Helen Vu 
aaronbradleysmith@gmail.com helenhvu@yahoo.com 
Portland State University 
USP 565 
Fall Quarter, December 2009
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 2 
Abstract 
Intersections and left-turn maneuvers have been identified as having the highest rates for negative 
bicycle and automobile interactions. Our study determines the need for additional transition and 
left-turn facilities along the eastbound Hawthorne bike corridor to access northbound residential 
feeder routes. The following questions framed our research: What are bicyclists’ perceptions of 
safety and comfort when performing a left-hand turn? Are they employing the suggested north-south 
bicycle route, if not, why? The data used in this study was collected from three bicycle counts 
and responses from 118 intercept surveys of existing users on the bike corridor. Survey results reveal 
that 61% of riders find merging across Hawthorne during peak traffic flows to be “very 
challenging.” If left-turn treatments were added to the intersection at Southeast 7th and Southeast 
Hawthorne, 71% would perceive an increase in their sense of safety, and 50% of cyclists traveling 
north indicate that they would modify their route in order to use this facility. This illustrates high 
user demand for left-turn treatments. The results of this study may be used to set priorities in the 
creation of a low-stress bicycle network by city and transportation designers, planners, and 
engineers.
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 3 
Introduction 
According to the Census Bureau’s American Communities Survey (2008), Portland, Oregon retains 
the highest bicycle ridership in the country, with 6.4% of work commuters utilizing the bicycle as 
their primary mode of transportation—a 52% jump from the prior year (as cited in “Portland Bike, 
Pedestrian, and Transit,” 2009). These cycling rates in Portland have been achieved through the 
city’s commitment to provide “low-stress, efficient, and comfortable facilit[ies].” The city reinforces 
this idea in its action plan to “ensure all neighborhoods have adequate low-stress bicycle facilities 
connecting to neighborhood commercial corridors and centers so that local residents can safely and 
comfortably access them by bicycle or on foot” (Portland Bicycle Plan for 2030 [PBP 2030], 2009, p. 
III & A-2). 
The Hawthorne Bridge over the Willamette River serves as a primary gateway for bicyclists 
traveling from downtown Portland into the city’s southeastern neighborhoods. At rush hour, as 
cyclists journey eastward from the bridge along this designated bike corridor, commuters with 
destinations to the north must merge across three lanes of heavy traffic in order to execute a left-hand 
turn. This maneuver involves multiple points of conflict between bicyclists and motorists, 
creating a potentially hazardous condition. This study examines the possible need for additional 
transition facilities along the eastbound Hawthorne bike corridor (Hawthorne bridge to Southeast 
12th Avenue) to allow safe access to northbound residential feeder routes, in keeping with the stated 
goals of Portland’s Bicycle Plan for 2030. 
In order to assess this need the following questions were posed: What are cyclists’ perceived 
levels of ease and safety when navigating this corridor and performing a left-hand turn? Are these 
cyclists employing the suggested north-south bike route that intersects this section of Hawthorne 
and if not, where are they turning and why? How safe is the left-hand turn maneuver onto this 
route? This study addresses these questions both through the perceptions of cyclists currently
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 4 
commuting along this corridor and through quantitative observations of existing conditions. The 
resulting data may be a valuable resource to planners and engineers in identifying the necessity of 
and demand for improved level of service at this location, and by cycling advocates who may lobby 
for such improvements. 
Literature Review 
To bolster the reasoning behind PBP’s focus on the creation of a low-stress bicycle network, this 
review will focus on the high conflict area of intersections and the maneuvering of left-hand turns. 
For the purposes of this study, the preceding topics will be examined through relevant data on 
facility safety and treatment, and will be limited to the modal perspective of the basic cyclist. A 
spotlight within these bounds will help to promote PBP 2030 objectives. 
Hazard Measurements: Numerous studies have combined crash statistics, roadway conditions, 
and user data in order to analyze leading points of conflict between cyclists and vehicles. A nine-year 
study conducted by Wachtel and Lewiston (1994) indicated that the majority of bicycle-motor 
vehicle accidents occurred at intersections. They found that intersections made up 74% of all 
bicycle-vehicle collisions (Wachtel and Lewiston, 1994). Data from Oregon’s Traffic Crash 
Summary (2009) support this finding. The 2008 Portland pedalcycle summary shows that 196 out of 
265 crashes took place at intersections, accounting for 74% of incidents that year (Oregon 
Department of Transportation, 2009, p. 82). This number becomes more astounding when one 
considers that only 10-20% of bicycle crashes are severe enough to be reported to officials (Portland 
Office of Transportation, 2007). 
It’s no wonder that even in European cities, where there has been a long history of bicycling 
ingrained in the culture of day-to-day living, that the European Conference of Ministers of 
Transport (ECMT) identifies the crossings of intersections and executions of left turns as hazardous
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 5 
to cyclists (ECMT, 2000, p. 28). Although it is not clear where crash reports and hospital trauma 
statistics overlap, trauma records may provide greater insight on bicycle facility safety. In 2005, 
Oregon hospitals had 297 patients from motor vehicle-pedalcyclist collisions; 3.7% of those patients 
died from injuries sustained during the conflict (Trauma System and Patient Profile, 2005). As noted 
in the Federal Highway Administration’s (FHWA) BIKESAFE, a 1996 study by Hunter et al. was 
able to make a reliable distribution of bicycle crash types. Primary causes of automobile-bicycle 
crashes were attributed to the following: 
• A motorist failing to yield (21.7%) 
• A bicyclist failing to yield at an intersection (16.8%) 
• A motorist turning or merging into the path of the bicyclist (12.1%) 
• A bicyclist failing to yield at midblock location (11.7%) 
• A motorist overtaking a bicyclist (8.6%) 
• A bicyclist turning or merging into the path of the motorist (7.3%) (as cited in 
FHWA BIKESAFE, n.d.) 
Contradicting measurements may be discovered when comparing various crash datasets. This is due 
to the lack of uniformity of crash-reporting procedures across agencies. A January 2006 report from 
the Portland Police Bureau implied that “cyclists merging into travel lanes” was the highest causal 
factor of fatalities in bicycle-automobile crashes at 28% (Improving Bicycle Safety, 2007). Utilizing 
methodologies to compute bicycle safety provides an additional option to gauge the cycling climate 
of an area, and specifically an intersection. Rating models like the Bicycle Safety Index Rating and 
Bicycle Level of Service have an intersection component, but fail to incorporate information about 
crashes and conflicts. The FHWA also describes a more comprehensive rating tool, the Bicyclist 
Intersection Safety Index (Bike ISI). This tool incorporates crash data, conflict and avoidance 
maneuvers, and subjective intersection ratings in its rating of intersections (FHWA, n.d.).
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 6 
Gains in Safety: Despite the number of injuries and fatalities reported by Oregon’s crash reports, 
Portland’s overall bicycle crash rate is decreasing, likely due to the exponential surge in bicycle 
ridership. Development of the bicycle network, in the form of intersection and left-turn facility 
upgrades, are consistent with PBP 2030 goals. Birk and Gellar (2005) state that engineers and 
planners must construct facilities to the highest standards in order to increase cyclist’s ease of use, 
and to minimize negative bicycle-automobile interactions. They have also effectively coined 
Portland’s “build it and they will come” approach to describe the city’s success in increasing bicycle 
use (Birk & Gellar, 2005). It is reported that over half of Portland residents limit their bicycling due 
to traffic safety concerns. Yet multiple studies have shown that increases in bicycling activity itself 
create safer conditions. In Copenhagen’s Cycle Policy (2002-2012), the report specifies that a 
“critical mass of cyclists decreases accident risk because motorists become more aware of cyclists, 
but also because more cyclists are indicative of a well-developed cycling network” (Nelson & 
Scholar, n.d., p. 14). 
Development of the bicycle network becomes more significant when one considers the three 
categories of cyclists: A—advanced, B—basic, and C—children. Group A cyclists (10% of overall 
cycling population) are more skilled and therefore more willing to use streets with or without bicycle 
facilities in their desire to travel on the most direct route. Group B cyclists (40% of biking 
population) are moderately skilled, have basic knowledge, and tend to bike occasionally, but prefer 
routes with bicycle facilities. For group C (10% of population), biking or public transportation is 
often their only modes of transportation because they are too young to drive (Allen, Rouphail, 
Hummer, & Milazzon, 1998, p. 29). This means that the greatest potential for new cyclists will come 
from group B. In order to attract these new bicyclists, Portland’s bicycle network must continue to 
improve bicyclists’ conflict areas. The “subjective feelings of comfort and safety will determine 
stress levels experienced, as well as the likelihood of traveling on specific routes or using specific
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 7 
travel modes” (Dewar & Olson, 2002, p. 597). Rietveld and Daniel found that overall perceptions of 
bicycle facilities play a significant role in the decision to bicycle. Boslaugh et al. discovered an inverse 
relationship between individual’s perceived safety risk and neighborhood bicycle ridership —the 
higher the perception of risk, the lower the bicycling levels (as cited in Sener, Eluru, & Bhat, 2008, p. 
2). This underscores the necessity to increase the safety of intersection and left-turn treatments. 
Innovative Treatments: In their 2004 paper, “Safety Mega Issue,” the Institute of Transportation 
Engineers (ITE) highlights the organization’s focus on the “design, construction, operation, and 
maintenance of intersections for safety of all user groups.” By considering the needs of all users, 
ITE’s improvements to intersection safety would contribute to making communities more livable 
and sustainable. In order to produce this outcome and maximize resources, ITE stresses the 
necessity for comprehensive research on the effectiveness of road countermeasures and bicycle 
treatments (Safety Mega Issue, 2004). There are few bicycle treatments, and even less research, 
which specifically address the challenge of intersections and left turns. The use of a colored bicycle 
lane through an intersection is one technique to tackle this issue. 
Colored bicycle lane marking through intersection 
These brightly painted lanes guide cyclists’ movements through intersections and caution motorists 
to conflicts with bicycle crossings. In a study of these treatments at intersections in Copenhagen,
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 8 
Jensen concluded that crash rates were reduced by 10% at sites with one demarcated bicycle lane. 
However, if more than one colored bike lane were present at an intersection crash rates would 
increase (Weigand, 2008, p.3). This highlights the importance of balancing the needs of all users by 
designing complete streets. 
Portland’s Platinum Bicycle Master Plan Process identified the intersections of Southeast 
Hawthorne and 7th, and Southeast Hawthorne and 11th, as two of Portland’s fourteen most difficult 
intersections to navigate as a bicyclist. The plan targeted these cumbersome intersections for 
immediate treatment (Improving Bicycle Safety, 2007). Examples of some possible designs to 
remedy the difficulty of left-hand turns include: 
• Two-Stage Signalized Left Turn: aka Copenhagen-left, or Jug-handle turn 
In the Copenhagen-left treatment, cyclists turn right at intersecting street and position themselves in 
front of traffic at cross street. On green light, they proceed straight. It is the “go right to turn left” 
design (PBP 2030: Appendix D, 2009, p. 26). This provides a safe way for cyclists to turn left from 
the right lane—this treatment is commonly found in European cities.
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 9 
• Two Bike Lane Transition 
This treatment decreases the number of merge maneuvers required to transition to the opposite 
bikeway facility (San Francisco Department of Parking and Traffic, 2003). The use of color for the 
left bike lane would warn motorists of high conflict points with left-turning cyclists, and would 
designate preferred left-turn routes in the bicycle network. 
• Streaming Bike Box/Advanced Stop Line (ASL) 
(San Francisco Department of Parking and Traffic, 2003).
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 10 
Pictured on the left: of SE 39th & SE Clinton, Portland, Oregon (Improving Bicycle Safety, 2007) 
Pictured on the right: The Netherlands, bike box with marked turn lanes 
Three studies in the UK; Allen, Bygrave et al., Wall, Davies et al., and Wheeler; found bike 
boxes/ASLs effective for allowing bicyclists to position themselves in front of stopped vehicles at 
signalized intersections. According to Allen et al. and Wall et al., this position reduces conflicts with 
turning motorists. Cyclists perceived an increase in visibility and safety. Research in the UK and a 
study in Eugene, Oregon, noted problems with vehicle encroachment in bike boxes, but hopefully 
signage and education may remedy this issue (as cited in Weigand, 2008, p. 5). 
• Bicycle Activated Signals: Scramble Traffic Light and Bicycle Only Signal Phase
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 11 
This treatment stops all motor vehicle movement at the intersection, provides an exclusive phase for 
bikes and pedestrians, and eliminates two-stage crossings (Walker, Tresidder, & Birk, 2009, p. 42 & 
48). In Portland, Oregon, Wolfe et al. collected data before and after an intersection was installed 
with a bicycle scramble light. Their results indicate the intersection had an increase of bicycle usage 
after the treatment was installed. In addition, illegal intersection crossings dropped from 78.1% to 
4.2% (as cited in Weigand, 2008, p. 8). In a study conducted in Davis, California, research showed 
that integrating a bicycle-only phase into the existing intersection signal minimized conflicts amongst 
bicyclists and motor vehicles. In a cost-benefit analysis, the study found that savings in crash 
reductions paid for the cost of signal installation (Korve and Niemeir, 2002). 
Methods 
In order to comprehensively assess the Southeast Hawthorne bike corridor’s adequate facilitation of 
left turns onto the Southeast 7th Avenue corridor, and thus determine the need and priority for 
additional facilities, this study will analyze three significant indicators: 
• Quantitatively evaluated safety of the bicycle left-hand turn maneuver from eastbound 
Southeast Hawthorne onto northbound Southeast 7th Ave., including its merge component 
(from right-hand bike lane to left lane). 
• User comfort and perception of safety in performing this maneuver. 
• Demand for additional left-hand turn facilities at this intersection. 
The methods described hereafter were chosen to evaluate these indicators by synthesizing objective 
data from traffic counts and an inventory of facility characteristics with qualitative data from user 
intercept surveys, and supplementing these measures though existing statistical data where 
appropriate.
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 12 
Bicyclist Intersection Safety Index: Commonly referred to as “Bike ISI,” this tool consists of a 
set of models developed in 2006 by the University of North Carolina’s Pedestrian and Bicycle 
Information Center to rate the safety of approach legs and movements through intersections 
(Carter, Hunter, Zegeer, Stewart, & Huang, 2006). Using extensively gathered crash data, behavioral 
data (conflicts and avoidance maneuvers), and subjective ratings from expert users at 67 sites across 
the U.S. (thirteen of which, incidentally, were in Portland), researchers developed a formula that 
indexes safety to physical intersection characteristics (Carter et al., 2006). The calculation method for 
Bike ISI, utilized in this study for a left turn from the right-hand bike lane of Southeast Hawthorne 
onto Southeast 7th Ave., is described in Carter, Hunter, Zegeer, and Stewart’s Pedestrian and Bicyclist 
Intersection Safety Indices: User Guide (2007). Inventory of the intersection will be taken from the 
following geometric and operational characteristics (brackets indicate corresponding formula input 
values): 
• Presence of bicycle lane on main street (Hawthorne). [“presence”: 0=no, 1=yes; “absence”: 
1=yes, 0=no] 
• Average daily traffic volume on main street. [ADT in thousands] 
• Number of through vehicle lanes on cross street (7th Ave.). [1, 2…] 
• Number of traffic lanes bicyclists must cross to make left turn. [1, 2…] 
• Speed limit on main street. [0=less than 35mph, 1=greater than or equal to 35mph] 
• Presence of on-street parking on main street approach. [0=no, 1=yes] 
• Presence of traffic signal at intersection. [0=no, 1=yes] 
These data will then be plugged into the following formula: 
Bike ISI = 1.100 + 0.025(main street ADT) + 0.836(bike lane presence) + 
0.485(traffic signal presence) + 0.736(main street speed limit * bike lane presence) + 
0.380(# of traffic lanes to cross * bike lane absence) + 0.200(on-street parking) 
The calculated index value will occur on a range between 1 and 6, with 1 representing a relatively 
safe intersection and 6 an unsafe intersection.
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 13 
Bike ISI was chosen for this study as a quantitative measure of safety due in large part to its 
unique ability to evaluate a specific approach and turn maneuver at a given intersection. Though 
previously developed methodologies do exist for measuring the “bicycle friendliness” of a given 
street or intersection (e.g. Bicycle Compatibility Index, Bicycle Level of Service), these all assume a 
continuous and straight line of bicycle travel, with no specific attention given to approaches or turns 
(Carter et al., 2006). Additionally, Bike ISI is the only model that incorporates crash data or 
behavioral data into its calculations, significantly increasing its objectivity and accuracy in measuring 
true safety. Though it possesses great advantages over other models of bicycle compatibility with 
respect to this study, Bike ISI was developed primarily as a comparative tool for ranking an 
intersection’s relative safety against others across a city (Carter et al., 2006). Applied only to an 
isolated location, the index value loses its local comparative significance and therefore becomes 
more abstract in its indication of favorability. However, Carter et al. surveyed an intentionally 
diverse array of sites in their pilot study (2006), so its results should form a useful baseline for 
general comparison. The results of that study were categorized for comparison according to right-turn, 
left-turn, and straight-through movements. The ratings for right turn and straight-through 
movements were clustered at the lower end of the scale in a tighter distribution than left turns, 
which were grouped in the middle of the scale across a wider range, indicating a lower average level 
of safety but greater variation between sites (Carter et al., 2006). 
Demand for additional left-turn facilities at Southeast 7th Ave. will be estimated through a 
combination of user preference survey and manual user counts along existing facilities. The user 
count portion will quantify the number of cyclists currently performing a left turn at 7th Ave. and at 
relatively interchangeable locations nearby. The likelihood of these other cyclists to use 7th Ave. 
after a left-turn-friendly redesign at its intersection with Hawthorne will be demonstrated in the 
responses of survey participants from that group when asked whether they would modify their route
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 14 
to use 7th Ave. given such a redesign. A resulting estimation of demand emerges when these survey 
responses are viewed in proportion to the number of cyclists observed turning after 7th. The 
integration of observed data with user preference survey results is recognized in the Federal 
Highway Administration’s Guidebook on Methods to Estimate Non-Motorized Travel: Overview of Methods 
(1999) as significantly advantageous for predicting user behavior. 
User Counts: A team of two data collectors will be stationed at the northeast corner of 9th and 
Hawthorne, one facing westward and the other facing eastward. Each data collector will be given a 
bicycle count form to record his or her observations (Appendix A). The westward-facing observer 
will tally (by street) each left-turning bicyclist as they cross the observer’s sidewalk line of sight on 
7th, 8th, and 9th Avenues. The eastward-facing observer will do the same for bicyclists turning left 
onto 10th and 12th Avenues (11th being a one-way southbound). These observation points are 
uniquely effective due to their relative lack of interference from car traffic (scanning across a 
sidewalk as opposed to a street), and also to the fact that the heaviest bike traffic occurs on the street 
furthest from the observer, maintaining a line of vision well-suited to the observation of multiple 
streets. Three separate two-hour counts will be conducted during afternoon peak hours (4-6pm) on 
a Tuesday, Wednesday, or Thursday, as these days are not shown to be statistically unique from one 
another (Alta, 2009). 
A number of logical assumptions constitute the rationale for the user count site selections. 
Southeast 7th Ave. is the only designated north-south bike route intersecting the Hawthorne bike 
route east of the bridge. Given that the intersection of these two streets is an acknowledged point of 
difficulty for cyclist navigation (Portland Office of Transportation, 2007), and that the eastbound 
Hawthorne bike corridor ends at 12th Ave., it stands to reason that cyclists encountering difficulty 
with the left turn at 7th would likely turn instead at 8th, 9th, 10th, or 12th. It is also likely that a 
significant number of Group A cyclists continuing their northward journey on 12th Ave. prefer this
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 15 
route, given its status as an arterial, but the allowance of five additional blocks for merging left may 
play a role in that decision. Manual user counts are a more sensible option for this study than 
automated counts due to their higher level of accuracy and lower cost (Schneider, Patten, and Toole, 
2005). The labor intensity of manual user counts can be a drawback in situations involving 
numerous count locations, but this study’s need for only two observers working only three hours 
each does not raise that concern. 
The City of Portland conducts annual bicycle counts throughout the city and publishes their 
aggregated results each year, along with a summary discussion of notable trends. Their counts are 
performed by 24-hour automated means in some locations and manually using peak flow 
extrapolation at many others, including the four main Willamette River bridges (Portland Bicycle 
Counts, 2007). Their 2007 study utilized 60 volunteers to conduct 93 counts across the city, among 
98 sites selected. With a scope this broad, they were able to analyze detailed bicycle usage trends on 
many different scales, from individual intersections to the city on the whole. The 2007 study 
concluded that Portland’s bicycle use is increasing rapidly, and doing so at an increasing pace. 
User Intercept Survey: The quantitative measures of safety and demand outlined to this point 
comprise the important empirical component of this study, but they do not paint a complete picture 
without the incorporation of subjective user input. An intercept survey facilitates a targeted 
evaluation of users’ perceived comfort and level of safety. Demand for additional facilities can be 
gauged more precisely through the directly stated preferences of potential users. For this study a user 
intercept study will be conducted among eastbound cyclists at the southeast corner of Southeast 6th 
Ave. and Southeast Hawthorne, shortly before they would reach the intersection at Southeast 7th 
Ave. In order to maximize the sample of these cyclists, a large sign will be placed in their line of 
sight one block west of the survey point (at Southeast Grand Ave. and Southeast Hawthorne) urging 
their participation. A second sign will be placed at the location of the survey, in clear view of cyclists
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 16 
approaching in the bike lane. Given that the parallel parking strip between Hawthorne’s bike lane 
and south curb does not allow parking on this portion of the block, it makes for a safe, highly 
visible, and unobtrusive location for cyclists to dismount before receiving their questionnaires. There 
is sufficient space here to accommodate a sizeable number of bicycles, and cyclists can transition to 
this spot from the bike lane without facing bus or vehicular conflicts. 
The user intercept survey method was selected for this study due to its effectiveness at 
isolating and targeting the desired sample. Its time and day of occurrence will mimic that of the user 
count in order to maximize the correspondence of their data sets. Depending on the number of 
cyclists who choose to participate, attracting a sample size sufficient to achieve a maximum 
acceptable sampling error of plus or minus 10% with a 95% confidence level may or may not require 
more than one two-hour survey period. Ideally the survey will achieve a much lower sampling error 
in one period, but due to the time and weather constraints surrounding this study, conducting the 
sample across more than one period could be relatively burdensome. A tightly constrained sampling 
error, however, is not necessary to adequately serve this study’s purpose of establishing a general 
idea of demand. 
An estimated count of daily peak-flow bicycle traffic near the survey location (population 
size) will be used to calculate sampling error as represented in Table 8 of the Transportation 
Cooperative Research Program’s TCRP Synthesis 63: On-board and Intercept Transit Survey Techniques 
(2005). The Portland Department of Transportation recently observed a summer average of 7,379 
bicycles per day on the Hawthorne bridge. Multiplying this count by a factor of 0.2 generally yields 
an accurate 4-6pm peak-flow number (Geller, 2008), most of whom are presumably traveling east 
and therefore exiting the bridge into our survey area. Certainly an unknown portion the bicyclists in 
that count are traveling west, but this will only cause the actual sampling error to be lower than 
calculated. The peak-flow number will be multiplied again by a factor of 0.5 to approximate the
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 17 
difference between a summer bicycle count and a winter count (Geller, 2009), assuming ridership 
levels on the survey date in November are similar to those during the winter. The high population 
number generated by not excluding westbound cyclists will be offset to some degree if the number 
of cyclists on the date of the survey is greater than that of an average day during the winter. By the 
calculation method described here, the population size for the survey is estimated at 738. 
Though effective in its ability to target a specific group of users for detailed questioning, the 
principle weaknesses of the user intercept survey as it pertains to this study lie in the time and labor 
intensity required to gather sufficiently accurate data, and the difficulty in estimating an accurate 
population size from which to calculate sampling error. 
In April 2006 a study seeking to understand cyclist use of off-road paths was undertaken in 
Melbourne, Australia, taking the form of an intercept survey coupled with a postage-paid self-completion 
questionnaire (Rose, 2007). Systematic sampling was used to select one out of every four 
counted cyclists for an intercept interview, in which they were asked to answer a few brief questions 
and given the take-home questionnaire to be returned via mail. In the end only one in seven actually 
completed the interview process, but of these an impressive 77% completed and mailed back the 
questionnaire. A detailed set of statistics on off-road trail use was gleaned from the survey’s
VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 18 
respondents, 85% of whom were commuters. The take-home-and-return questionnaire was 
demonstrated here as a potentially effective way to augment the data gathered in an intercept survey. 
The methodology outlined in this study for examining the question “Is there a need for 
additional bicycle left-turn facilities at Southeast Hawthorne Blvd. and Southeast 7th Ave.?” matches 
a pertinent selection of variables to a balanced set of established measurement techniques. A 
synthesis of the resulting data will provide an appropriate indicator for evaluating this question. 
Results and Interpretations 
The data of primary interest in this study were gathered from the user intercept survey and are 
augmented by the manual user counts and Bicyclist Intersection Safety Index. The user intercept 
survey was conducted during the afternoon peak period of 4-6pm on two consecutive days, both 
during relatively cold and overcast weather conditions. Daylight was present only during the first 30- 
40 minutes of the survey period. Responses were collected from a total of 118 bicyclists out of the 
estimated population size of 738, yielding a sampling error of ±8.3% at a 95% confidence level 
(assuming a 50% sample proportion). Respondents were asked eleven questions, three regarding 
bicycle usage characteristics, four regarding perception of safety and merge behavior in the study’s 
location of interest, two determining the bicyclist’s current trip characteristics, and two regarding 
attitudes toward a redesign of the intersection of Southeast 7th Ave. and Southeast Hawthorne. Five 
of these questions were directed only at bicyclists turning left on their current trip, yet many others 
still responded to them. Due to the questions of consistency that this raised, these responses were 
removed from consideration.

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VolatileEndeavors-SmithVu

  • 1. Volatile Endeavors: Negotiating Left Turns Along the SE Hawthorne Bike Corridor Aaron Smith Helen Vu aaronbradleysmith@gmail.com helenhvu@yahoo.com Portland State University USP 565 Fall Quarter, December 2009
  • 2. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 2 Abstract Intersections and left-turn maneuvers have been identified as having the highest rates for negative bicycle and automobile interactions. Our study determines the need for additional transition and left-turn facilities along the eastbound Hawthorne bike corridor to access northbound residential feeder routes. The following questions framed our research: What are bicyclists’ perceptions of safety and comfort when performing a left-hand turn? Are they employing the suggested north-south bicycle route, if not, why? The data used in this study was collected from three bicycle counts and responses from 118 intercept surveys of existing users on the bike corridor. Survey results reveal that 61% of riders find merging across Hawthorne during peak traffic flows to be “very challenging.” If left-turn treatments were added to the intersection at Southeast 7th and Southeast Hawthorne, 71% would perceive an increase in their sense of safety, and 50% of cyclists traveling north indicate that they would modify their route in order to use this facility. This illustrates high user demand for left-turn treatments. The results of this study may be used to set priorities in the creation of a low-stress bicycle network by city and transportation designers, planners, and engineers.
  • 3. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 3 Introduction According to the Census Bureau’s American Communities Survey (2008), Portland, Oregon retains the highest bicycle ridership in the country, with 6.4% of work commuters utilizing the bicycle as their primary mode of transportation—a 52% jump from the prior year (as cited in “Portland Bike, Pedestrian, and Transit,” 2009). These cycling rates in Portland have been achieved through the city’s commitment to provide “low-stress, efficient, and comfortable facilit[ies].” The city reinforces this idea in its action plan to “ensure all neighborhoods have adequate low-stress bicycle facilities connecting to neighborhood commercial corridors and centers so that local residents can safely and comfortably access them by bicycle or on foot” (Portland Bicycle Plan for 2030 [PBP 2030], 2009, p. III & A-2). The Hawthorne Bridge over the Willamette River serves as a primary gateway for bicyclists traveling from downtown Portland into the city’s southeastern neighborhoods. At rush hour, as cyclists journey eastward from the bridge along this designated bike corridor, commuters with destinations to the north must merge across three lanes of heavy traffic in order to execute a left-hand turn. This maneuver involves multiple points of conflict between bicyclists and motorists, creating a potentially hazardous condition. This study examines the possible need for additional transition facilities along the eastbound Hawthorne bike corridor (Hawthorne bridge to Southeast 12th Avenue) to allow safe access to northbound residential feeder routes, in keeping with the stated goals of Portland’s Bicycle Plan for 2030. In order to assess this need the following questions were posed: What are cyclists’ perceived levels of ease and safety when navigating this corridor and performing a left-hand turn? Are these cyclists employing the suggested north-south bike route that intersects this section of Hawthorne and if not, where are they turning and why? How safe is the left-hand turn maneuver onto this route? This study addresses these questions both through the perceptions of cyclists currently
  • 4. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 4 commuting along this corridor and through quantitative observations of existing conditions. The resulting data may be a valuable resource to planners and engineers in identifying the necessity of and demand for improved level of service at this location, and by cycling advocates who may lobby for such improvements. Literature Review To bolster the reasoning behind PBP’s focus on the creation of a low-stress bicycle network, this review will focus on the high conflict area of intersections and the maneuvering of left-hand turns. For the purposes of this study, the preceding topics will be examined through relevant data on facility safety and treatment, and will be limited to the modal perspective of the basic cyclist. A spotlight within these bounds will help to promote PBP 2030 objectives. Hazard Measurements: Numerous studies have combined crash statistics, roadway conditions, and user data in order to analyze leading points of conflict between cyclists and vehicles. A nine-year study conducted by Wachtel and Lewiston (1994) indicated that the majority of bicycle-motor vehicle accidents occurred at intersections. They found that intersections made up 74% of all bicycle-vehicle collisions (Wachtel and Lewiston, 1994). Data from Oregon’s Traffic Crash Summary (2009) support this finding. The 2008 Portland pedalcycle summary shows that 196 out of 265 crashes took place at intersections, accounting for 74% of incidents that year (Oregon Department of Transportation, 2009, p. 82). This number becomes more astounding when one considers that only 10-20% of bicycle crashes are severe enough to be reported to officials (Portland Office of Transportation, 2007). It’s no wonder that even in European cities, where there has been a long history of bicycling ingrained in the culture of day-to-day living, that the European Conference of Ministers of Transport (ECMT) identifies the crossings of intersections and executions of left turns as hazardous
  • 5. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 5 to cyclists (ECMT, 2000, p. 28). Although it is not clear where crash reports and hospital trauma statistics overlap, trauma records may provide greater insight on bicycle facility safety. In 2005, Oregon hospitals had 297 patients from motor vehicle-pedalcyclist collisions; 3.7% of those patients died from injuries sustained during the conflict (Trauma System and Patient Profile, 2005). As noted in the Federal Highway Administration’s (FHWA) BIKESAFE, a 1996 study by Hunter et al. was able to make a reliable distribution of bicycle crash types. Primary causes of automobile-bicycle crashes were attributed to the following: • A motorist failing to yield (21.7%) • A bicyclist failing to yield at an intersection (16.8%) • A motorist turning or merging into the path of the bicyclist (12.1%) • A bicyclist failing to yield at midblock location (11.7%) • A motorist overtaking a bicyclist (8.6%) • A bicyclist turning or merging into the path of the motorist (7.3%) (as cited in FHWA BIKESAFE, n.d.) Contradicting measurements may be discovered when comparing various crash datasets. This is due to the lack of uniformity of crash-reporting procedures across agencies. A January 2006 report from the Portland Police Bureau implied that “cyclists merging into travel lanes” was the highest causal factor of fatalities in bicycle-automobile crashes at 28% (Improving Bicycle Safety, 2007). Utilizing methodologies to compute bicycle safety provides an additional option to gauge the cycling climate of an area, and specifically an intersection. Rating models like the Bicycle Safety Index Rating and Bicycle Level of Service have an intersection component, but fail to incorporate information about crashes and conflicts. The FHWA also describes a more comprehensive rating tool, the Bicyclist Intersection Safety Index (Bike ISI). This tool incorporates crash data, conflict and avoidance maneuvers, and subjective intersection ratings in its rating of intersections (FHWA, n.d.).
  • 6. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 6 Gains in Safety: Despite the number of injuries and fatalities reported by Oregon’s crash reports, Portland’s overall bicycle crash rate is decreasing, likely due to the exponential surge in bicycle ridership. Development of the bicycle network, in the form of intersection and left-turn facility upgrades, are consistent with PBP 2030 goals. Birk and Gellar (2005) state that engineers and planners must construct facilities to the highest standards in order to increase cyclist’s ease of use, and to minimize negative bicycle-automobile interactions. They have also effectively coined Portland’s “build it and they will come” approach to describe the city’s success in increasing bicycle use (Birk & Gellar, 2005). It is reported that over half of Portland residents limit their bicycling due to traffic safety concerns. Yet multiple studies have shown that increases in bicycling activity itself create safer conditions. In Copenhagen’s Cycle Policy (2002-2012), the report specifies that a “critical mass of cyclists decreases accident risk because motorists become more aware of cyclists, but also because more cyclists are indicative of a well-developed cycling network” (Nelson & Scholar, n.d., p. 14). Development of the bicycle network becomes more significant when one considers the three categories of cyclists: A—advanced, B—basic, and C—children. Group A cyclists (10% of overall cycling population) are more skilled and therefore more willing to use streets with or without bicycle facilities in their desire to travel on the most direct route. Group B cyclists (40% of biking population) are moderately skilled, have basic knowledge, and tend to bike occasionally, but prefer routes with bicycle facilities. For group C (10% of population), biking or public transportation is often their only modes of transportation because they are too young to drive (Allen, Rouphail, Hummer, & Milazzon, 1998, p. 29). This means that the greatest potential for new cyclists will come from group B. In order to attract these new bicyclists, Portland’s bicycle network must continue to improve bicyclists’ conflict areas. The “subjective feelings of comfort and safety will determine stress levels experienced, as well as the likelihood of traveling on specific routes or using specific
  • 7. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 7 travel modes” (Dewar & Olson, 2002, p. 597). Rietveld and Daniel found that overall perceptions of bicycle facilities play a significant role in the decision to bicycle. Boslaugh et al. discovered an inverse relationship between individual’s perceived safety risk and neighborhood bicycle ridership —the higher the perception of risk, the lower the bicycling levels (as cited in Sener, Eluru, & Bhat, 2008, p. 2). This underscores the necessity to increase the safety of intersection and left-turn treatments. Innovative Treatments: In their 2004 paper, “Safety Mega Issue,” the Institute of Transportation Engineers (ITE) highlights the organization’s focus on the “design, construction, operation, and maintenance of intersections for safety of all user groups.” By considering the needs of all users, ITE’s improvements to intersection safety would contribute to making communities more livable and sustainable. In order to produce this outcome and maximize resources, ITE stresses the necessity for comprehensive research on the effectiveness of road countermeasures and bicycle treatments (Safety Mega Issue, 2004). There are few bicycle treatments, and even less research, which specifically address the challenge of intersections and left turns. The use of a colored bicycle lane through an intersection is one technique to tackle this issue. Colored bicycle lane marking through intersection These brightly painted lanes guide cyclists’ movements through intersections and caution motorists to conflicts with bicycle crossings. In a study of these treatments at intersections in Copenhagen,
  • 8. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 8 Jensen concluded that crash rates were reduced by 10% at sites with one demarcated bicycle lane. However, if more than one colored bike lane were present at an intersection crash rates would increase (Weigand, 2008, p.3). This highlights the importance of balancing the needs of all users by designing complete streets. Portland’s Platinum Bicycle Master Plan Process identified the intersections of Southeast Hawthorne and 7th, and Southeast Hawthorne and 11th, as two of Portland’s fourteen most difficult intersections to navigate as a bicyclist. The plan targeted these cumbersome intersections for immediate treatment (Improving Bicycle Safety, 2007). Examples of some possible designs to remedy the difficulty of left-hand turns include: • Two-Stage Signalized Left Turn: aka Copenhagen-left, or Jug-handle turn In the Copenhagen-left treatment, cyclists turn right at intersecting street and position themselves in front of traffic at cross street. On green light, they proceed straight. It is the “go right to turn left” design (PBP 2030: Appendix D, 2009, p. 26). This provides a safe way for cyclists to turn left from the right lane—this treatment is commonly found in European cities.
  • 9. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 9 • Two Bike Lane Transition This treatment decreases the number of merge maneuvers required to transition to the opposite bikeway facility (San Francisco Department of Parking and Traffic, 2003). The use of color for the left bike lane would warn motorists of high conflict points with left-turning cyclists, and would designate preferred left-turn routes in the bicycle network. • Streaming Bike Box/Advanced Stop Line (ASL) (San Francisco Department of Parking and Traffic, 2003).
  • 10. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 10 Pictured on the left: of SE 39th & SE Clinton, Portland, Oregon (Improving Bicycle Safety, 2007) Pictured on the right: The Netherlands, bike box with marked turn lanes Three studies in the UK; Allen, Bygrave et al., Wall, Davies et al., and Wheeler; found bike boxes/ASLs effective for allowing bicyclists to position themselves in front of stopped vehicles at signalized intersections. According to Allen et al. and Wall et al., this position reduces conflicts with turning motorists. Cyclists perceived an increase in visibility and safety. Research in the UK and a study in Eugene, Oregon, noted problems with vehicle encroachment in bike boxes, but hopefully signage and education may remedy this issue (as cited in Weigand, 2008, p. 5). • Bicycle Activated Signals: Scramble Traffic Light and Bicycle Only Signal Phase
  • 11. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 11 This treatment stops all motor vehicle movement at the intersection, provides an exclusive phase for bikes and pedestrians, and eliminates two-stage crossings (Walker, Tresidder, & Birk, 2009, p. 42 & 48). In Portland, Oregon, Wolfe et al. collected data before and after an intersection was installed with a bicycle scramble light. Their results indicate the intersection had an increase of bicycle usage after the treatment was installed. In addition, illegal intersection crossings dropped from 78.1% to 4.2% (as cited in Weigand, 2008, p. 8). In a study conducted in Davis, California, research showed that integrating a bicycle-only phase into the existing intersection signal minimized conflicts amongst bicyclists and motor vehicles. In a cost-benefit analysis, the study found that savings in crash reductions paid for the cost of signal installation (Korve and Niemeir, 2002). Methods In order to comprehensively assess the Southeast Hawthorne bike corridor’s adequate facilitation of left turns onto the Southeast 7th Avenue corridor, and thus determine the need and priority for additional facilities, this study will analyze three significant indicators: • Quantitatively evaluated safety of the bicycle left-hand turn maneuver from eastbound Southeast Hawthorne onto northbound Southeast 7th Ave., including its merge component (from right-hand bike lane to left lane). • User comfort and perception of safety in performing this maneuver. • Demand for additional left-hand turn facilities at this intersection. The methods described hereafter were chosen to evaluate these indicators by synthesizing objective data from traffic counts and an inventory of facility characteristics with qualitative data from user intercept surveys, and supplementing these measures though existing statistical data where appropriate.
  • 12. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 12 Bicyclist Intersection Safety Index: Commonly referred to as “Bike ISI,” this tool consists of a set of models developed in 2006 by the University of North Carolina’s Pedestrian and Bicycle Information Center to rate the safety of approach legs and movements through intersections (Carter, Hunter, Zegeer, Stewart, & Huang, 2006). Using extensively gathered crash data, behavioral data (conflicts and avoidance maneuvers), and subjective ratings from expert users at 67 sites across the U.S. (thirteen of which, incidentally, were in Portland), researchers developed a formula that indexes safety to physical intersection characteristics (Carter et al., 2006). The calculation method for Bike ISI, utilized in this study for a left turn from the right-hand bike lane of Southeast Hawthorne onto Southeast 7th Ave., is described in Carter, Hunter, Zegeer, and Stewart’s Pedestrian and Bicyclist Intersection Safety Indices: User Guide (2007). Inventory of the intersection will be taken from the following geometric and operational characteristics (brackets indicate corresponding formula input values): • Presence of bicycle lane on main street (Hawthorne). [“presence”: 0=no, 1=yes; “absence”: 1=yes, 0=no] • Average daily traffic volume on main street. [ADT in thousands] • Number of through vehicle lanes on cross street (7th Ave.). [1, 2…] • Number of traffic lanes bicyclists must cross to make left turn. [1, 2…] • Speed limit on main street. [0=less than 35mph, 1=greater than or equal to 35mph] • Presence of on-street parking on main street approach. [0=no, 1=yes] • Presence of traffic signal at intersection. [0=no, 1=yes] These data will then be plugged into the following formula: Bike ISI = 1.100 + 0.025(main street ADT) + 0.836(bike lane presence) + 0.485(traffic signal presence) + 0.736(main street speed limit * bike lane presence) + 0.380(# of traffic lanes to cross * bike lane absence) + 0.200(on-street parking) The calculated index value will occur on a range between 1 and 6, with 1 representing a relatively safe intersection and 6 an unsafe intersection.
  • 13. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 13 Bike ISI was chosen for this study as a quantitative measure of safety due in large part to its unique ability to evaluate a specific approach and turn maneuver at a given intersection. Though previously developed methodologies do exist for measuring the “bicycle friendliness” of a given street or intersection (e.g. Bicycle Compatibility Index, Bicycle Level of Service), these all assume a continuous and straight line of bicycle travel, with no specific attention given to approaches or turns (Carter et al., 2006). Additionally, Bike ISI is the only model that incorporates crash data or behavioral data into its calculations, significantly increasing its objectivity and accuracy in measuring true safety. Though it possesses great advantages over other models of bicycle compatibility with respect to this study, Bike ISI was developed primarily as a comparative tool for ranking an intersection’s relative safety against others across a city (Carter et al., 2006). Applied only to an isolated location, the index value loses its local comparative significance and therefore becomes more abstract in its indication of favorability. However, Carter et al. surveyed an intentionally diverse array of sites in their pilot study (2006), so its results should form a useful baseline for general comparison. The results of that study were categorized for comparison according to right-turn, left-turn, and straight-through movements. The ratings for right turn and straight-through movements were clustered at the lower end of the scale in a tighter distribution than left turns, which were grouped in the middle of the scale across a wider range, indicating a lower average level of safety but greater variation between sites (Carter et al., 2006). Demand for additional left-turn facilities at Southeast 7th Ave. will be estimated through a combination of user preference survey and manual user counts along existing facilities. The user count portion will quantify the number of cyclists currently performing a left turn at 7th Ave. and at relatively interchangeable locations nearby. The likelihood of these other cyclists to use 7th Ave. after a left-turn-friendly redesign at its intersection with Hawthorne will be demonstrated in the responses of survey participants from that group when asked whether they would modify their route
  • 14. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 14 to use 7th Ave. given such a redesign. A resulting estimation of demand emerges when these survey responses are viewed in proportion to the number of cyclists observed turning after 7th. The integration of observed data with user preference survey results is recognized in the Federal Highway Administration’s Guidebook on Methods to Estimate Non-Motorized Travel: Overview of Methods (1999) as significantly advantageous for predicting user behavior. User Counts: A team of two data collectors will be stationed at the northeast corner of 9th and Hawthorne, one facing westward and the other facing eastward. Each data collector will be given a bicycle count form to record his or her observations (Appendix A). The westward-facing observer will tally (by street) each left-turning bicyclist as they cross the observer’s sidewalk line of sight on 7th, 8th, and 9th Avenues. The eastward-facing observer will do the same for bicyclists turning left onto 10th and 12th Avenues (11th being a one-way southbound). These observation points are uniquely effective due to their relative lack of interference from car traffic (scanning across a sidewalk as opposed to a street), and also to the fact that the heaviest bike traffic occurs on the street furthest from the observer, maintaining a line of vision well-suited to the observation of multiple streets. Three separate two-hour counts will be conducted during afternoon peak hours (4-6pm) on a Tuesday, Wednesday, or Thursday, as these days are not shown to be statistically unique from one another (Alta, 2009). A number of logical assumptions constitute the rationale for the user count site selections. Southeast 7th Ave. is the only designated north-south bike route intersecting the Hawthorne bike route east of the bridge. Given that the intersection of these two streets is an acknowledged point of difficulty for cyclist navigation (Portland Office of Transportation, 2007), and that the eastbound Hawthorne bike corridor ends at 12th Ave., it stands to reason that cyclists encountering difficulty with the left turn at 7th would likely turn instead at 8th, 9th, 10th, or 12th. It is also likely that a significant number of Group A cyclists continuing their northward journey on 12th Ave. prefer this
  • 15. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 15 route, given its status as an arterial, but the allowance of five additional blocks for merging left may play a role in that decision. Manual user counts are a more sensible option for this study than automated counts due to their higher level of accuracy and lower cost (Schneider, Patten, and Toole, 2005). The labor intensity of manual user counts can be a drawback in situations involving numerous count locations, but this study’s need for only two observers working only three hours each does not raise that concern. The City of Portland conducts annual bicycle counts throughout the city and publishes their aggregated results each year, along with a summary discussion of notable trends. Their counts are performed by 24-hour automated means in some locations and manually using peak flow extrapolation at many others, including the four main Willamette River bridges (Portland Bicycle Counts, 2007). Their 2007 study utilized 60 volunteers to conduct 93 counts across the city, among 98 sites selected. With a scope this broad, they were able to analyze detailed bicycle usage trends on many different scales, from individual intersections to the city on the whole. The 2007 study concluded that Portland’s bicycle use is increasing rapidly, and doing so at an increasing pace. User Intercept Survey: The quantitative measures of safety and demand outlined to this point comprise the important empirical component of this study, but they do not paint a complete picture without the incorporation of subjective user input. An intercept survey facilitates a targeted evaluation of users’ perceived comfort and level of safety. Demand for additional facilities can be gauged more precisely through the directly stated preferences of potential users. For this study a user intercept study will be conducted among eastbound cyclists at the southeast corner of Southeast 6th Ave. and Southeast Hawthorne, shortly before they would reach the intersection at Southeast 7th Ave. In order to maximize the sample of these cyclists, a large sign will be placed in their line of sight one block west of the survey point (at Southeast Grand Ave. and Southeast Hawthorne) urging their participation. A second sign will be placed at the location of the survey, in clear view of cyclists
  • 16. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 16 approaching in the bike lane. Given that the parallel parking strip between Hawthorne’s bike lane and south curb does not allow parking on this portion of the block, it makes for a safe, highly visible, and unobtrusive location for cyclists to dismount before receiving their questionnaires. There is sufficient space here to accommodate a sizeable number of bicycles, and cyclists can transition to this spot from the bike lane without facing bus or vehicular conflicts. The user intercept survey method was selected for this study due to its effectiveness at isolating and targeting the desired sample. Its time and day of occurrence will mimic that of the user count in order to maximize the correspondence of their data sets. Depending on the number of cyclists who choose to participate, attracting a sample size sufficient to achieve a maximum acceptable sampling error of plus or minus 10% with a 95% confidence level may or may not require more than one two-hour survey period. Ideally the survey will achieve a much lower sampling error in one period, but due to the time and weather constraints surrounding this study, conducting the sample across more than one period could be relatively burdensome. A tightly constrained sampling error, however, is not necessary to adequately serve this study’s purpose of establishing a general idea of demand. An estimated count of daily peak-flow bicycle traffic near the survey location (population size) will be used to calculate sampling error as represented in Table 8 of the Transportation Cooperative Research Program’s TCRP Synthesis 63: On-board and Intercept Transit Survey Techniques (2005). The Portland Department of Transportation recently observed a summer average of 7,379 bicycles per day on the Hawthorne bridge. Multiplying this count by a factor of 0.2 generally yields an accurate 4-6pm peak-flow number (Geller, 2008), most of whom are presumably traveling east and therefore exiting the bridge into our survey area. Certainly an unknown portion the bicyclists in that count are traveling west, but this will only cause the actual sampling error to be lower than calculated. The peak-flow number will be multiplied again by a factor of 0.5 to approximate the
  • 17. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 17 difference between a summer bicycle count and a winter count (Geller, 2009), assuming ridership levels on the survey date in November are similar to those during the winter. The high population number generated by not excluding westbound cyclists will be offset to some degree if the number of cyclists on the date of the survey is greater than that of an average day during the winter. By the calculation method described here, the population size for the survey is estimated at 738. Though effective in its ability to target a specific group of users for detailed questioning, the principle weaknesses of the user intercept survey as it pertains to this study lie in the time and labor intensity required to gather sufficiently accurate data, and the difficulty in estimating an accurate population size from which to calculate sampling error. In April 2006 a study seeking to understand cyclist use of off-road paths was undertaken in Melbourne, Australia, taking the form of an intercept survey coupled with a postage-paid self-completion questionnaire (Rose, 2007). Systematic sampling was used to select one out of every four counted cyclists for an intercept interview, in which they were asked to answer a few brief questions and given the take-home questionnaire to be returned via mail. In the end only one in seven actually completed the interview process, but of these an impressive 77% completed and mailed back the questionnaire. A detailed set of statistics on off-road trail use was gleaned from the survey’s
  • 18. VOLATILE ENDEAVORS: NEGOTIATING LEFT TURNS Smith & Vu 18 respondents, 85% of whom were commuters. The take-home-and-return questionnaire was demonstrated here as a potentially effective way to augment the data gathered in an intercept survey. The methodology outlined in this study for examining the question “Is there a need for additional bicycle left-turn facilities at Southeast Hawthorne Blvd. and Southeast 7th Ave.?” matches a pertinent selection of variables to a balanced set of established measurement techniques. A synthesis of the resulting data will provide an appropriate indicator for evaluating this question. Results and Interpretations The data of primary interest in this study were gathered from the user intercept survey and are augmented by the manual user counts and Bicyclist Intersection Safety Index. The user intercept survey was conducted during the afternoon peak period of 4-6pm on two consecutive days, both during relatively cold and overcast weather conditions. Daylight was present only during the first 30- 40 minutes of the survey period. Responses were collected from a total of 118 bicyclists out of the estimated population size of 738, yielding a sampling error of ±8.3% at a 95% confidence level (assuming a 50% sample proportion). Respondents were asked eleven questions, three regarding bicycle usage characteristics, four regarding perception of safety and merge behavior in the study’s location of interest, two determining the bicyclist’s current trip characteristics, and two regarding attitudes toward a redesign of the intersection of Southeast 7th Ave. and Southeast Hawthorne. Five of these questions were directed only at bicyclists turning left on their current trip, yet many others still responded to them. Due to the questions of consistency that this raised, these responses were removed from consideration.