The outline of today’s presentation is… I will start with the introduction, describe the data collection sites for this study, the methodology, and then present the results of objective and the subjective evaluations conducted for the project. Mojtaba will help me with presentation of some of the results.The presentation will end with conclusions and recommendations for future research.
This study reviewed the literature on work zone speed limits and it was found that drivers do not comply with the posted speed limits. Drivers compliance reduces if work zone speed limits are lowered by more than 10 mph from the original speed limit. Further, compliance is even lower when unreasonably low speed limits are posted. Work zone speed limits are particularly difficult to enforce and follow as different wz speed limits even in the same state are used. The posted work zone speed limits are dependent on work zone construction activity. The most commonly posted speed limits are 0, 5, 10 and 20 mph.
The objectives of this project were to examine work zone speed limit, its impact on vehicle speeds and drivers’ compliance with the posted speed limits both quantitatively and qualitatively. To evaluate quantitatively, objective evaluation was carried out by collecting and analyzing work zone speed data. This study examined the impacts of lane closure, lane width reduction, and work zone activity on speed of vehicles. To evaluate qualitatively, two surveys were conducted. A drivers’ assessment of work zone speed limit and drivers’ compliance with the speed limit were carried out. US DOTs were also surveyed to get an insight on the different speed limit strategies in work zones and their common practices.The results of this study can be beneficial to the work zone design practices as one of the primary objectives of work zones design and control is to improve the safety of work zones.
The data collection sites were selected in consultation with the Missouri DOT. Data on 5 different sites were collected and all sites were located on sections of interstate I-44, a four lane highway. The data were collected near Waynesville, Rolla, Cuba, and Pacific in Missouri. In Pacific, data were collected on both the EB and WB sections of I-44. In total, 53 hours of data were collected from five sites.For all sites, the highway speed limit was 70 mph and the original lane width was assumed to be 12-13 ft. The heavy vehicle percentage was around 25% for all sites.The work zone speed limit for the Waynesville and the Rolla sites was 60 mph. For the waynesville site the work activity was pavement reconstruction and for the rolla site it was pavement rehabilitation, both major construction activity. For the rolla site, the width of the open lane was reduced by tabular markers.
For the Pacific site, data was collected for both the EB and the WB directions, the speed limit was 50 mph lower than other sites. The construction activity was addition of a new median lane. The lane width was measured to be 10 ft, reduced by pavement markings. Traffic volumes were also higher compared to other sites. At this Pacific site, we also surveyed the drivers.
For the Cuba site, we collected data for 5 hours, the speed limit was 60 mph. The construction activity was rumble striping. The table shows the summary of work zone data sites. Hold the slides for like 15 seconds.
The driver survey data were collected at Rolla and Pacific work zones. A gas station close to the work zone was selected for data collection.In total 118 truck and passenger car drivers were surveyed. The drivers were surveyed on their preference of location of work zone signs, their perception of work zone speed limit, compliance with posted speed limit, and effect of construction activity on their behavior.
The driver were male (75%) and female (25%) drivers and all were asked to complete a paper and pencil survey. The participants were from all ages; from less than 20 years old to more than 70 years old.About 70% of the drivers were passenger car drivers and the rest were heavy vehicle drivers.Driving experience of drivers were almost uniformly distributed among the 4 categories; less than 5 years of experience to more than 20 years of experience.
To evaluate the common work zone practices of the DOTs, an online survey was carried out. It was sent to 50 states and 27 states responded. The survey posed questions on work zone speed management policies, the nature of work activities that warrant reduced speed limits, factors that determine the posted speed limit, the type of signs used to post speed limits, compliance levels, and the measures taken to evaluate the effectiveness of different strategies.
Work zone field data were collected using high definition video cameras. The collected data were analyzed using a machine vision software called Autoscope that extracted vehicle speed with an accuracy of +1 to -1 mph. The speeds were calibrated and validated using a sample speed data collected using laser speed guns on the field.Vehicles in the traffic stream were identified as free flowing based on 5 second headway. 5- sec headway was used as the vehicles with time headway less than 5 seconds were deemed to be in a platoon and their speeds affected by leading vehicles. Free flow speed was used in this study as it represents the desired speed of drivers not affected by the leading vehicle, and can be used in the evaluation of work zone speed limit complianceFFS were assumed to be normally distributed and because of the relative insensitivity of t-test to normal distribution it was used in speed evaluation.The drivers’ surveyed collectedat two sites near the work zones were analyzed to examine their preferences. The DOT data were analyzed to study the best practice of different US DOTs.
The work zone field data were extracted using Autoscoope. Autoscopeis a video image processing software that uses user-defined sensors on a snapshot of a video made at the site. The various types of sensors placed on the screen detect the changes in the streaming video. Sensors that were used in the study included the movement, count, and speed detectors.The data from the sensors (detectors) were tolled automatically as well as manually and the volume, speed, and headway of the vehicles were extracted from the output file for the duration of the study.
This slide shows the statistical analysis performed on the extracted data.The first test was to evaluate if the differences between the mean (two sample t-test) of cars and trucks, null hypothesis was that speed of cars is less than speed of trucks and alternate hypothesis was speed of cars is more than speed of trucks. The second test evaluated the effect of construction activity on the mean speeds of vehicles, The third test statistically examinedthe effects of lane closure on vehicles, speed of vehicles when 2 lanes were open were compared with the speed when one lane was open.And the last test was used to evaluate the compliance of cars and truck drivers speed with the posted speed limit.Also, F-tests were used to compare the variation in speed between cars and trucks, and between the construction and no construction period.
This slide shows the result of the mean and standard deviation of speed for cars and trucks for work zones during construction activity. blue colo shows the mean speed and red shows the stdvSpeed limit for Waynesville and Rolla was 60 mph and 50 mph for Pacific site, it can be seen that Mean speeds were higher than the speed limit except for the Rolla site. The values in rectangles show the difference between the speed of cars and trucks. It can be seen that For all WZ sites, the mean speed of cars was higher than trucks.Passenger car speeds were statistically higher than the truck speeds except for Waynesville, that could be because of the small sample size that reduced the power of test to determine the significanceStandard deviation of the vehicle speeds varied from the lowest value of 4.1 mph for cars for the eastbound Pacific to the highest value of 6.4 for trucks in Rolla site-F-tests showed that generally, variation in speed was lower during construction activity compared to no construction activity
This slide also shows the speed characteristics but for no construction period. As can be seen Cuba site is also included here.Once again, Mean speeds were higher than the speed limit except for the Rolla site, that might be because of the reduced lane width using tubular markersStandard deviation of the vehicle speeds varied from the lowest value of 4.1 mph for trucks for the eastbound Pacific to the highest value of 7.5 mph for trucks Waynesville WBthe effect of lane closure on speed was also conducted for Pacific site and it was found that with no lane closure (2 lanes open) drivers drove statistically faster.
The effect of construction activity on speeds were statistically tested.Differences of the speeds with construction and no construction (right two columns of table) were statistically significant at a confidence level of 99% (bold), indicating that construction activity prompts drivers to reduce their speeds significantly. -The standard deviation for no construction period was higher than construction period (left two columns); however, this difference was statistically significant for the Pacific westbound site and only for cars on the Pacific eastbound site
This slide shows the speed compliance test for work zones during construction activity period. Speed limit for Waynesville and Rolla was 60 mph, and for pacific was 50 mph. blue colors show the percentage of vehicles driving faster than speed limit, red color shows the percentage of drivers traveling more than 5 mph above speed limit and the green color is for more than 10 mph above speed limit.It can be observed from the figure that vehicles generally traveled faster than the speed limit.The percentage of drivers traveling faster than the speed limit was lower compared to no construction activity that will be seen in the next slide. The rectangles show the difference of mean speed with the speed limit.For the Rolla site with a lane width reduced by tubular markers, car and truck speeds were statistically lower than the posted speed limit at a 99% confidence level.Car and truck speeds for the Waynesville, Cuba, and Pacific sites were statistically higher than the speed limit at a 99% confidence level.when the speed limit was lower, 50-mph, the percentage of drivers traveling above the speed limit was generally higher compared to when the speed limit was higher, 60-mph
This slide also shows the speed compliance test for work zones but during no construction activity periodAgain, It can be observed from the figure that vehicles generally traveled faster than the speed limit.With no construction activity, more than 10% of the drivers traveled faster than 10-mph above the speed limit. This amplifies the effects of construction activity in reducing the drivers’ speeds in work zones.For the Rolla site with a lane width reduced by tubular markers, car and truck speeds were statistically lower than the posted speed limit at a 99% confidence level.Car and truck speeds for the Waynesville, Cuba, and Pacific sites were statistically higher than the speed limit at a 99% confidence level.The work zone reduced speed limit sign was not really effective in reducing vehicle speeds to below the speed limit especially with no construction activity.
Alright now for the results of the subjective evaluation
This slide presents the drivers’ preference of the location of different signs, the three colors show the three work zone signs “work zone ahead”, “lane closure ahead”, and “the speed limit sign”. The numbers for every color adds up to 100%.It can be observed that about 80% of drivers prefer to know about the presence of a work zone more than 3 miles ahead, but there is not much difference in their preference for seeing the “lane closed ahead” sign and “reduced speed ahead” sign. Although, they prefer to see these signs 2-3 miles ahead.Only 13% of respondents preferred to see the “Left/Right Lane Closed Ahead” sign 0.5-1 mile before the work zone; Hardly any of the participants indicated a desire to see the “Speed Limit 50/60 mph” sign less than 0.5 mile before the work zone, and more than 75% preferred to see it more than a mile before the work zone.
This slide presents the lane changing behavior of drivers before the construction zone. More than 50% of drivers indicated they change their lane right after the “lane closure” sign. Slightly more than 30% of drivers indicated they change their lane before they see the work zone. Around 10% of drivers change their lane after they see the work zone. Less than 5% indicated that they would change their lane right before the work zone taper. Result of this survey shows that there are very few aggressive drivers on the road but these results also require field data collection to verify this finding. Current result shows that most drivers driver safely to prevent any hazardous conditions.
This question evaluated the perception of drivers about speed limit compliance of other drivers.The result shows the compliance for the two speed limits, 60 mph for Rolla site and 50 mph for the Pacific site.For both speed limits, 60 to 84% of drivers agreed that other people follow the speed limit. Truck drivers perception of other drivers’ compliance of the speed limit was more positive compared to car drivers.These results compared with the speed limit compliance from the objective analysis shows that (for the Rolla site) when people are driving below the speed limit they overestimate other drivers’ speed. For the Pacific site, where most people drove above the speed limit, they underestimated other drivers’ speed.
This slide shows the travelers’ driving speed through the work zone per their response.This question was posed to see how drivers evaluate their own speed. Their speed compared to the speed limit reveals the level of speed limit compliance in the eyes of the drivers. For the Pacific site, most of the car drivers stated that they drove above the speed limit but most truck drivers stated that they drove below the speed limit. For the Rolla site most of the car and truck drivers indicated that they drove below the speed limit.
The speed limit suggested by drivers is shown in this slide. The suggested speed limit reflects the appropriateness of the fixed speed limits designated for the work zones.As can be observed, for the pacific site, since most of drivers were driving above the speed limit, 93% of car drivers and 73% of truck drivers suggested a speed limit higher than 50 mph.On the other hand, for the Rolla site, since most of them were driving below the speed limit because of reduced lane width, they saw the speed limit appropriate and suggested speed limits below 60 mph which was the actual speed limit. This shows that drivers want a speed limit according to their driving speed.
In addition to the previous slides that documented drivers responses using graphs and tables, drivers were also surveyed about the effects of construction activity on their driving speed.Their opinion about the safety of work zone speed limit were surveyed, and if they had any delays traveling through the work zone.Majority of drivers (92%) indicated that construction activity reduces their speed.Most of the drivers (80%) found the speed limit to be safe to travel through the work zone, and 60% of them indicated travel delay through the work zone, most of which were from the Rolla site survey where they were traveling below the speed limit.
The US DOT responded in terms of major factor that determine the speed limit as the construction crew, lane width, the roadway geometry and the type of construction activity. Most states indicated a maximum reduction of 10 mph in the speed limit through the work zone. Most states used static speed limits and the best strategy for speed limit compliance is the presence of highway patrol. Twenty five percent of the states found regulatory speed limit signs to be effective.
In terms of what we concluded, We found passenger cars speeds higher than speeds of trucks by 1-2 mph.We found that construction activity significantly effects speeds. Speed of both passenger cars and trucks reduce by 2-5 mph.With no-work zone construction activity, or construction activity not close to the open lane, more than 10% drivers traveled higher than 10 mph above the wz speed limitWe found low drivers’ compliance with the posted wz speed limit. In terms of reduced lane width as a result of tubular markers, we found significant reduced in speed of vehicles. The speed of vehicles reduced by 8 mph below the posted speed limit.
We also found that more than 90% of drivers agreed that construction activity reduced their speeds and this was in agreement with the results of the field data examined. Drivers overestimate the number of other drivers that donot comply with the speed limit when most of them were traveling below the speed limit.Additionally, drivers underestimate the number of other drivers that do not comply with the speed limit when most of them were traveling above the speed limit .
When asked on suggested about the speed limit, drivers suggested a speed limit that is consistent with their own speed irrespective of the existing speed limit, roadway or environmental factors.Drivers prefer to be well informed of a work zone more than a mile ahead of the work zone
Majority of drivers suggested reduction in speed limit in work zones which consistently had congested conditions.Further, majority of drivers suggested increasing the speed limit for work zones with a lower limit when the work zone did not witness congested conditions consistently.This study, therefore, strongly recommends the use of variable speed limits for work zones with short-term construction periods. Speed limits can be adjusted real time based on the traffic conditions and the construction activity. This will help with mobility and safety of traffic during off-peak hours and when the work zone is not active.This study recommends the use of tubular markers rather than pavement marking for separating the construction area from the traveling lanes.
The authors acknowledge the research grants from the Smart Work Zone Deployment Initiative administered by the Iowa State University and the Mid-America Transportation Center at the University of Nebraska. They also appreciate the cooperation and assistance with data collection from Ms. Victoria Woods, Mr. Tim Hellebusch and their staff at the Missouri DOT. Finally, the authors thank Dr. Praveen Edara from the University of Missouri at Columbia for providing the results of the state DOT survey.
Vehicle Speeds in Work Zones: An Objective and Subjective Analysis
Vehicle Speeds in Work Zones: An Objective and Subjective Analysis Mojtaba Ale Mohammadi, PhD Candidate Ghulam H. Bham, PhD Civil, Architectural and Environmental Engineering Missouri University of Science and Technology
DISCLAIMER The contents of this report reflect the views of the authors, who are responsible for the facts and the accuracy of the information presented herein. This document is disseminated under thesponsorship of the Department of Transportation University Transportation Centers Program, in the interest of information exchange. The U.S.Government assumes no liability for the contents or use thereof.
OUTLINE• Introduction• Data Collection Sites• Methodology• Results –Objective Evaluation –Subjective Evaluation• Conclusions and Recommendations 3
INTRODUCTION• Posted speed limits alone do not reduce vehicle speed• Drivers’ compliance reduces if speed limit is lowered by more than 10 mph or unreasonable speed limits• Most commonly used speed limit reductions are 0, 5, 10, and 20 mph• Different speed limits in work zones make enforcement difficult 4
OBJECTIVESObjective evaluation: vehicle speeds anddrivers’ speed limit complianceDrivers’ Assessment: drivers’ perceptionof speed limit and their complianceDOT Survey: work zone speed limit bestpractices 5
DATA COLLECTION SITES Waynesville I-44 West Bound, Right lane closed, Mile Marker: 152.8 August 13th, 2009 – Thursday 3 hours of data from 5 to 8 pm WZ Posted Speed Limit 60 mph Work Activity: Pavement Reconstruction Rolla I-44 West Bound, Right lane closed, Mile Marker: 185 October 2nd, 2009 – Thursday 5 hours of data from 12:15 to 7 pm WZ Posted Speed Limit 60 mph Work activity: Pavement Rehabilitation Lane width reduced by tubular markers 6
DATA COLLECTION SITES Pacific (WB & EB) I-44, Left lane closed, Mile Marker: 253 June 9th, 16th, 24th, 2010 – Wednesday, Wednesday, Thursday 20 hours of data from 6 to 11 am WZ Posted Speed Limit: 50 mph Work Activity: Addition of a New Lane Lane width reduced by pavement marking Data was also collected with no lane closure 7
DATA COLLECTION SITES Cuba I-44, West Bound, left lane closed, Mile Marker: 202.6 November 6th, 2009 – Thursday 5 hours from 11:30 to 4:30 pm WZ Posted Speed Limit 60 mph Work Activity: Rumble Striping Summary of work zone dataWork zone Lane closed Data WZ Speed limit ActivityWaynesville Right 3 hours 60 mph Pavement reconstructionRolla Right 5 hours 60 mph Pavement rehabilitationPacific (WB) Left 20 hours 50 mph Additional lanePacific (EB) Left 20 hours 50 mph Additional laneCuba Left 5 hours 60 mph Rumble striping 8
DRIVER SURVEY DATA• Collected at gas stations near work zones• Car and truck drivers surveyed: 118• Actual speed versus posted speed limit• Speed limit compliance of other drivers• Effect of construction activity on their speed• Preference for work zone signage 9
DRIVER SURVEY 18 Age distribution 16 14 12 10 Frequency 8 6 4 2 0 Age (years) Double <5 Trailer > 20 25% 7% 30% Passenger Single Car Trailer 69% 21% 5-10 Single 20% Unit RV 10-20 2% 1% 25% Driving experience (years) Vehicle Composition 10
DOT SURVEY DATATo identify common WZ practices of DOTS• WZ speed management policies• WZ activities that warrant a reduction in speedlimit• Factors influence WZ speed limit• Compliance levels 11
METHODOLOGYField Data Extraction-Sites were videotaped using HD video cameras-Autoscope system software used to extract vehicle free flow speeds (FFS)-FFS based on 5-second headway-FFS assumed to be normally distributed-Drivers were surveyed using a questionnaire near the Rolla and Pacific sites-DOTs were surveyed on their common practices related to work zone speed limits 12
METHODOLOGY Speed detector Movement detectors Count detector Autoscope software for extracting the vehicle speed 13
METHODOLOGYo Difference in speed of cars and truckso The effect of construction activity on vehicles speedo Effect of lane closure on vehicles (1-lane vs. 2-lane open)o Evaluate speed limit compliance 14
OBJECTIVE EVALUATION Characteristics of Vehicle Speed – During Construction 80 Difference in mean speeds 70 Standard Deviation Mean 5.8 6.3 60 Percentage (%) 6.1 5.4 5.3 4.1 4.5 50 6.4 40 1.2 mph 2.0 mph *** 1.8 mph *** 2.6 mph ** 30 20 10 0 Car Truck Car Truck Car Truck Car Truck Waynesville WB Rolla WB Pacific WB Pacific EB *** Significant at 99% level of confidence ** Significant at 95% level of confidence 16
OBJECTIVE EVALUATION Characteristics of Vehicle Speed – During No-Construction 80 Difference in mean speeds 70 7.0 7.5 Standard Deviation Mean 4.7 4.9 60 7.1 6.6 6.7 5.4Percentage (%) 6.4 4.1 50 40 3.9 mph *** 4.1 mph *** 2.2 mph *** 1.8 mph *** 3.0 mph*** 30 20 10 0 Car Truck Car Truck Car Truck Car Truck Car Truck *** Significant at 99% level of confidence ^ One lane open = 1, Two lanes open = 2 *** Significant at 99% level of confidence; ** Significant at 95% level of confidence; * Significant at 90% level of confidence 17
OBJECTIVE EVALUATIONEffects of Construction Activity on Vehicle Speeds Difference in Standard Difference in MeanWork Zone Deviation # (mph) # (mph)Site Passenger Truck Trucks Passenger Cars Cars sWaynesville 1.18 1.19 4.0 2.2WBRolla WB 0.96 0.01 4.5 3.0Pacific WB 1.21 1.42 3.5 3.6Pacific EB 1.32 -0.44 4.7 2.8# Difference = speed with no construction – speed with constructionBold: Significant at 99% level of confidence 18
OBJECTIVE EVALUATION Speed Limit Compliance – During Construction Above Speed Limit > 5 mph Above Speed Limit 80 +6.78*** 70 +2.11** +1.47* +9.11***Percentage (%) +1.78** +1.38* 60 50 40 30 20 10 -14.56*** 0 -11.85** Truck Truck Truck Car Truck Car Car Car *** Significant at 99% level of confidence; ** Significant at 95% level of confidence; * Significant at 90% level of confidence 19
OBJECTIVE EVALUATION Speed Limit Compliance – During No-Construction Above Speed Limit > 5 mph Above Speed Limit 100 +30.50*** 90 +9.74*** +1.43* +8.85*** 80 +8.91*** +9.63*** +4.5*** 70Percentage (%) +1.43* 60 50 -7.13*** 40 30 -10.16*** 20 10 0 Car Truck Car Truck Car Truck Car Truck Car Truck Waynesville WB Rolla WB Cuba WB Pacific WB Pacific EB Work Zones *** Significant at 99% level of confidence; ** Significant at 95% level of confidence; * Significant at 90% level of confidence 20
DRIVER SURVEY How many miles in advance would you prefer to know about the presence of a work zone? How many miles in advance would you like to see the “Lane Closed Ahead” sign? How many miles in advance would you like to see the “Reduced Speed Limit” sign? >3 80 23 18 2-3 8 37 36Miles 1-2 12 26 32 0.5-1 13 14 < 0.5 1 0 20 40 60 80 100 120 140 Percent Drivers’ preference regarding location of WZ signs 22
DRIVER SURVEY 60 50 40Percentage (%) 30 20 10 0 Right after the first Before I see the work After I see the work Just before the taper "Lane Closed Ahead" zone zone sign Where drivers move out of the closed lane? 23
DRIVER SURVEY 100 16 20 40 36 80 Percent 60 80 84 40 60 64 20 0 Car Truck Car Truck Disagree Pacific (50 mph) Rolla (60 mph) Agree Did other drivers follow the speed limit? 24
DRIVER SURVEY Drivers Speed 5 7 (mph) 100 12 20 >60 80 48 50-60Percent 48 60 84 40-50 53 30-40 40 38 21 <30 20 20 14 10 16 5 0 Car Truck Car Truck Pacific (50 mph) Rolla (60 mph) Drivers’ speed through the work zone 25
DRIVER SURVEY• 92% of total respondents indicated construction activity reduced their speed• 80% found the work zone speed limit to be safe• 60% of participants indicated travel delay 27
DOT SURVEY – WORK ZONE PRACTICES• Twenty seven states responded to the survey• Major factors in determining the speed limit: – presence of workers, lane width, – roadway alignment, and type of activity• Seventy percent indicated a maximum reduction of 10-mph• Static speed limit signs are most commonly used• Best strategy to increase compliance is use of law enforcement• Regulatory signs found effective by 25% of the respondents 28
CONCLUSIONS• Passenger cars speeds higher than trucks by 1-2 mph• Construction activity significant effects speed – Passenger cars and trucks speeds reduce by 2-5 mph• With no-work zone construction activity, or construction activity not close to the open lane – More than 10% drivers traveled higher than 10 mph above the speed limit• Low drivers’ compliance with static speed limit• Reduced lane width using tubular markers significantly affected the speed of vehicles – Speed reduced 8 mph below the speed limit 29
CONCLUSIONS• Drivers agree that construction activity reduces their speed• Field data showed construction reduces speed• When most drivers traveled below the speed limit – They overestimate the number of other drivers that do not comply with the speed limit• When most drivers traveled above the speed limit – They underestimate the number of other drivers that do not comply with the speed limit 30
CONCLUSIONS• Higher compliance if WZ speed limit are consistent with drivers expectation• Drivers prefer well informed more than a mile ahead of WZ 31
RECOMMENDATIONS• Vary WZ speed limits with varying levels of traffic congestion• Use of variable speed limits for work zones with closed lanes and short-term construction activity periods• To reduce speed, tubular markers than pavement markings for separating the closed lane (construction area) from the open lanes 32
ACKNOWLEDGEMENT• The research grant from the Smart Work Zone Deployment Initiative (SWZDI) and Mid- America Transportation Center (MATC)• Ms. Victoria Woods, Mr. Tim Hellebusch and their staff at the Missouri DOT in data collection• Praveen Edara at University of Missouri at Columbia for sharing the DOT survey results 33
You can copy any of these graphics and paste them on other slides.