3. To reduce the negative impact of traffic on highways, it is
necessary to adequately collect information that describes
the extent of the problems and identifies their locations.
Such information is usually collected by organizing and
conducting traffic surveys and studies.
The main traffic engineering studies are:
1. Spot speed studies.
2. Volume studies.
3. Travel time and delay studies.
4. Parking studies.
4. Speed is an important transportation consideration
because it relates to safety, time, comfort, convenience,
and economics.
Spot speed studies are conducted to estimate the
distribution of speeds of vehicles in a stream of traffic at a
particular location on a highway. They are used for:-
Establishing speed zones and determine speed trends
Peak flow analysis
Design geometric alignment.
Analyze accident data.
Evaluate the effects of physical improvements.
5. For a spot speed study at a selected location, a sample size of
at least 50 and preferably 100 vehicles is usually used (Ewing
1999)
Spot speed data are gathered using one of three methods:
A. Stopwatch method,
B. Radar meter method,
C. Pneumatic road tube method.
6. The stopwatch method is the least expensive and least accurate
method.
The stopwatch method can be used to successfully complete a
spot speed study using a small sample size taken over a
relatively short period of time.
A stopwatch spot speed study includes five key steps:
1. Obtain appropriate study length.
2. Select proper location and layout.
3. Record observations on stopwatch spot speed study data form.
4. Calculate vehicle speeds.
5. Generate frequency distribution table and determine speed
percentiles.
7. 1. Obtain Appropriate Study Length.
The study length is important because it is used in the calculation
of vehicle speeds. It is given in table below.
If these lengths are not appropriate, another length can be used
assuming it is long enough for reliable observer reaction times.
2. Select Proper Location and Layout.
When selecting a location and layout, care must be exercised so
that the observer can clearly see any vertical reference posts.
The observer should be positioned higher than the study area
and be looking down.
8. The position could be on a bridge or a roadway back slope.
The observer should use reference points to aid in collecting the
elapsed time it takes a vehicle to travel through the study area.
The reference point to start timing may for example be a brightly
colored vertical post.
The reference point to end timing may be a tree or a signpost in
the observer’s sight line.
An accurate sketch of the site should be documented, including
number of lanes, position of observer, and description of
reference points.
9. 3. Record Observations on Stopwatch Spot Speed Data Form.
On the stopwatch spot speed data form the observer records
the date, location, posted speed limit, weather conditions, start
time, end time, and down time.
As the front wheels of a vehicle (or only the lead vehicle in a
group) cross a mark or pavement crack at the beginning of the
predetermined study length, the observer starts the stopwatch.
The watch is stopped when the vehicle’s front wheels pass a
reference line in front of the observer. Record the elapsed time
observed.
4. Calculate Vehicle Speeds To calculate vehicle speed, the
formula given below in the (Robertson 1994):
Where
V = spot speed (mph), D = length (feet), and
T = elapsed time (seconds).
10. Example -1,
if the spot speed study length is 100 feet and the motorist’s
elapsed time is 2.5 seconds, the motorist is traveling at
speed of,
5.Generate Frequency Distribution Table and Determine Speed
Percentiles.
Determine the 50th and 85th speed percentiles using a
frequency distribution table and calculations as described
below.
Example – 2 Frequency distribution table is given below so
determine the 50th and 85th speed percentile.
The frequency of vehicles is the number of vehicles
recorded at each speed.
11.
12. The 50th percentile falls between 27 and 30 mph and the
85th percentile falls between 33 and 36 mph.
Since the sample size equals 100 vehicles, the cumulative
frequency and cumulative percent are the same.
As can be observed from table are not found in the
cumulative percent column.
So that the following equation is used for calculating
speed percentiles .
where SD = speed at PD , PD = percentile desired, Pmax =
higher cumulative percent, Pmin = lower cumulative
percent, Smax = higher speed, and Smin = lower speed.
13. The 50th percentile of speed (PD = 50%) falls between 27 and
30 mph. so Smax = 30 mph and Smin = 27 mph
The higher cumulative percent (Pmax) is 54%, and the lower
cumulative percent (Pmin) is 34%.
Therefore, to find SD at PD = 50%,
SD=[(50%-34%)/(54%-34%)*(30mph-27mph)]+27mph=29.4
The 85th percentile of speed (PD = 85%) falls between 33 and
36 mph . so Smax = 36 mph and Smin = 33 mph
The higher cumulative percent (Pmax) is 86%, and the lower
cumulative percent (Pmin) is 72%.
Therefore, to find SD at PD = 50%,
14. B. RADAR METER METHOD.
A radar meter is a commonly used device for directly
measuring speeds in spot speed studies.
This device may be hand-held, mounted in a vehicle, or
mounted on a tripod.
The effective measuring distance for radar meters ranges from
200 feet up to 2 miles (Parma,2001).
A radar meter requires line-of-sight to accurately measure
speed and is easily operated by one person.
If traffic is heavy or the sampling
strategy is complex, two radar
units may be needed.
15. Large vehicles such as trucks and buses send the strongest
return signal to the radar meters and as a result smaller
vehicles may not be detected.
If there is a presence of large vehicles, the observer may need
to record the speeds of vehicles that are alone.
16. A radar meter spot speed study includes four key steps:
1. Select proper location and placement of radar meter.
2. Determine an appropriate selection strategy.
3. Record observations on spot speed study data form.
4. Generate frequency distribution table and determine speed
percentiles.
1. Select Proper Location and Placement of Radar Meter
The positioning of the radar unit is determined by the
capabilities of the radar unit (as listed in the users’ manual).
The unit should also be concealed from the view of motorists.
Effective ranges may be up to 2 miles, but as the distance
increases the effectiveness decreases.
17. An accurate sketch of the site should be documented,
including number of lanes, position of observer, and
description of reference points.
2. Determine an Appropriate Selection Strategy
Except for studies conducted under low-volume conditions, it
is impossible to obtain a radar measurement for every vehicle.
For peak flow analysis, speeds are measured during the peak
period.
For assessing general speed trends or for setting speed limits,
off-peak measurements are more appropriate.
The selection of the target vehicle that represents the vehicle
population under study is also important.
A good question to ask is, “What type or types of vehicles are
of concern. cars, trucks, buses, or others?”
18. A random sample will reduce the tendency to select the
vehicles that stand out.
For example, the observer could obtain a speed reading from
every fourth vehicle or every tenth vehicle.
3. Record Observations on Radar Meter Spot Speed Data Form.
On the data form the observer records the date, location,
posted speed limit, weather conditions, start time, end time,
and down time.
A slash is recorded on the data form corresponding to
speed observed for each selected vehicle (or only the lead
vehicle in a group) under the appropriate vehicle-type
classification.
19. 4. Generate Frequency Distribution Table and Determine
Speed Percentiles
Determine the 50th and 85th speed percentiles using a
frequency distribution table and calculations as described
earlier.
See data recorded in data table form below
20.
21.
22.
23. For the 50th percentile of speed , PD = 50%, Pmax = 53%,
Pmin = 45%, Smax= 35 mph, and Smin = 34 mph, so
For the 85th percentile of speed , PD = 85%, Pmax = 88%,
Pmin = 81%, Smax= 41 mph, and Smin = 40 mph, so
24. C. PNEUMATIC ROAD TUBE METHOD
The pneumatic road tube method is normally used for longer
data collection time periods than those of either the stopwatch
or radar meter method.
Using this method, pneumatic tubes are placed in the travel lanes
and are connected to recorders located at the side of the road
Records
pneumatic road tubes
25. A pneumatic road tube spot speed study includes four key steps :
1. Perform necessary office preparations.
2. Install and calibrate data collection equipment.
- The separation of the pneumatic tubes should be 2–15 feet
3. Check data and retrieve equipment.
-The recorder first measures the elapsed time it takes the vehicle
to pass over the tubes. The elapsed time can be checked with a
stopwatch.
-Then this time interval is converted to the corresponding spot
speed.
4. Generate frequency distribution table and determine speed
percentiles.
-Determine the 50th and 85th speed percentiles using a
frequency distribution table and calculations as described earlier.
26. 2.3 TRAFFIC VOLUME STUDIES
Transport engineers often use counts of number of vehicles or
pedestrians passing a point, entering an intersection, or using a
particular facility such as travel lane, crosswalk or sidewalk.
Counts are usually samples of actual volumes, although
continuous counting is also sometimes performed.
Sampling periods may range from a few minutes to a month or
more.
Volume characteristics are expressed using:
1. Average Annual Daily Traffic (AADT)
2. Average Daily Traffic (ADT)
3. Peak Hour Volume (PHV)
4. Vehicle Classification (VC)
27. 1. Average Annual Daily Traffic (AADT)
The average of 24-hour counts collected every day of the
year.
vehicles per day (VPD) in both directions
Expressed as average of 24 hours counts for 365 days (1
year)
Used for determining and assigning traffic trends, feasibility
studies, etc
2. Average Daily Traffic (ADT)
The average of 24-hour counts collected over a
number of days greater than one but less than a year.
Vehicles per day (VPD) in both directions.
Used for planning, measurement of current demand, evaluate
traffic flow
28. 3. Peak Hour Volume (PHV) the maximum number of
vehicles that pas a point on a highway during a period of
60 consecutive minutes.
hour)
peak
within
minutes
15
during
(4)(Volume
Volume
Hour
Peak
Factor
Hour
Peak
)
(4)(PV
PHV
PHF
15
PHF
PHV
DHV
Volume
Hourly
Design
29. Example: The afternoon peak hour traffic flow data of traffic on
a major road are:
10:45pm-11:00pm 400 vehicles ,
11:00pm-11:15pm 700 vehicles
11:15pm-11:30pm 600 vehicles
11:30pm-11:45pm 400 vehicles
Data Analysis:
*PHV = 400 + 700 + 600 + 400 = 2100 vehicles
* PV15 = 700 vehicles ; *PHF = 2100/(4)(700) = 0.75
Thus, DHV=2100/0.75=2800 vehicles
30. 4. Vehicle Classification (VC) : traffic count is done with
respect to the type of vehicles. For example, according to
ERA, vehicles are classified into passenger cars, buses,
trucks, truck-trailers, and articulated trucks.
31. 2.4 Travel Time & Delay Studies.
Determines the amount of time required to travel from one point
to another on a given route.
Information may also be collected on the locations, durations,
and causes of delays.
Definition of Terms Related to travel time and Delay Studies
Travel time:- is the time taken by a vehicle to traverse a given
section of a highway without interruption.
Running time:- is the time a vehicle is actually in motion while
traversing a give section of a highway.
Delay:- is the time lost by a vehicle due to causes beyond the
control of the driver.
Operational delay:- is that part of the delay caused by the
impedance of other traffic.
Stopped-time delay :- is that part of the delay during which the
vehicle is at rest.
32. Fixed delay:- is that part of the delay caused by control devices such
as traffic signals.
Travel-time delay :- is the difference between the actual travel time
and the time that will be obtained by assuming that a vehicle
traverses the study section at an average speed equal to that for an
uncontested traffic flow on the section being studied.
Applications of Travel Time and Delay Data
Determination of the efficiency of a route with respect to its ability
to carry traffic.
Identification of locations with relatively high delays and the causes
for those delays.
Performance of before-and-after studies to evaluate the
effectiveness of traffic operation improvements
33. 2.5 Parking studies
There are two types of parking facilities
1. On-Street Parking Facilities:- These are also known as curb
facilities. Parking bays are provided alongside the curb on one
or both sides of the street.
2. Off-Street Parking Facilities:- These facilities may be privately
or publicly owned; they include surface lots and garages.
Definitions of Parking Terms
A space-hour:- is a unit of parking that defines the use of a
single parking space for a period of 1 hr.
Parking volume(parking accumulation):- is the total number of
vehicles that park in a study area during a specific length of
time, usually a day.
These data can be plotted as a curve of parking volume
(accumulation) against time, which shows the variation of the
parking accumulation during the day.
34. The parking load:- is the area under the accumulation curve
between two specific times.
Parking duration :- is the length of time a vehicle is parked at a
parking bay.
Parking turnover:- is the rate of use of a parking space. It is
obtained by dividing the parking volume for a specified period
by the number of parking spaces.
Example: The following data was collected at a certain parking area
with 20 parking spaces for a duration 12 days. Calculate the
parking volume and parking turnover.
Time
(days)
1 2 3 4 5 6 7 8 9 10 11 12
No. of
vehicles
parking
15 20 20 20 18 12 16 14 15 20 20 15
35. Solution: Parking volume=Total no. of vehicles/ No. of days
=205/12= 17 veh/day
Parking turnover= parking volume/no. of parking spaces
= 17/20= 0.85 veh/day/space
→ On street parking
