Access Management Techniques to Improve Traffic Operations and Safety: A Case Study of a Full vs. Directional Median Opening Sunanda Dissanayake Department of Civil Engineering Kansas State University 2118 Fiedler Hall Manhattan, KS 66506
[email protected] John Lu Department of Civil and Environmental Engineering University of South Florida 4202 E. Fowler Avenue, ENB118 Tampa, FL 33620
[email protected]
ABSTRACT Access management applications, which could effectively be used to improve traffic operations and safety, are getting increased popularity. While there are some studies providing important information on various access management methods and techniques, questions still remain surrounding the effects of specific access management treatments on roadway operations and safety. This study provides the results of a case study in one of such areas, where the operational and safety characteristics of a full median opening are compared with those of a directional median opening, in the form of a before-and-after study. Field data were collected at the site by using video camera technique. Before period consisted of one week of field data where the intersection operated as a full median opening. The median was then converted into a directional median opening by using temporary physical barriers and field data were collected again in the same way for another week. Only the data collected during daytime, under good weather conditions with no special events were considered in the analysis. Operational characteristics were measured in terms of weighted average delay and weighted average travel time experienced by the left turning vehicles from the stop controlled approach (driveway), where safety performance was measured in terms of number of conflicts and rate of conflicts. According to the findings, the total weighted average travel delay was significantly reduced after the median opening was made to function as directional. However, changes in the travel times were not statistically significant. The average number of conflicts per hour was reduced by almost 50 percent whereas conflict rate per thousand involved vehicles was also significantly reduced. Additionally, the severity of conflicts measured subjectively was also found to be reduced during the after time period. Accordingly, both traffic operations and safety situations were found to be improved when the full median opening was converted to a directional opening. Key words: access management—medians—traffic operations and safety
Proceedings of the 2003 Mid-Continent Transportation Research Symposium, Ames, Iowa, August 2003. © 2003 by Iowa State University. The contents of this paper reflect the views of the author(s), who are responsible for the facts and accuracy of the information presented herein.
INTRODUCTION Access management helps achieve the necessary balance between traffic movement and property access by careful control of the location, type, and design of driveways and street intersections (1). Applications in the area of access management are getting increasingly popular in many of the states throughout the United States. While there are some studies providing important information on various access management methods and techniques, questions still remain surrounding the effects of certain specific access management treatments on roadway operations and safety. One such area is operational and safety characteristics of full median openings versus those of directional median openings. Current practice adopted by the Florida Department of Transportation (FDOT) is to have directional median openings on major arterials where the design speed is greater than 40 mph. Accordingly FDOT is considering the replacement of some of the full median openings with directional median openings, which sometimes receives criticism from the general public and the abutting commercial developments. Lack of more quantified results and documentation available for presenting the benefits of directional median openings in such situations was evident at public hearings, which was the main motivation towards this project. This paper describes a case study on the subject in the form of a “before and after” study, which was a part of a bigger project in which right turn followed by U-turn from driveways was considered as an alternative to direct left turn from driveways. The main objective of this case study was to compare safety and operational impacts of converting a full median opening located on a major arterial into a directional median opening. As a result of this conversion left turning traffic from the driveway was forced to make right turns and then U-turns at the next available median opening or signalized intersection. METHODOLOGY The Site The site considered in this study is located on U.S. 19 and 115th Avenue, in Pinellas County, Florida. Layout of the study site is illustrated in Figure 1. U.S.19 is a major arterial oriented in the north-south direction, with three and four lanes on the southbound and northbound, respectively. The northbound and southbound lanes are separated by a raised median. The posted speed limit on this segment of the road is 55 mph. This site was the subject of a geometric improvement related to left-turn movements. Initially, during the before period, the median was a full median opening that allowed vehicles to turn either to the left directly or to make the right turn first and then make a U-turn at a median opening located 400 feet from the street in order to travel northbound. The median opening was approximately 120 feet in length, which allowed three or more vehicles to wait on the median storage while impending the movement of other vehicles. Then, the median opening was closed and converted into a directional median opening, so that vehicles departing from the side street could only turn right. A descriptive analysis of the traffic conflict data collected at this site was performed because this site allowed the possibility to examine and evaluate the implications of changing a full median opening to a directional median opening in the frame of a before and after study (2).
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U.S. 19
Gas Station 600 feet
115th Street 420 feet
1620 feet
FIGURE 1. Layout of the Study Site: U.S. 19 at 115th Street
Change During the before period, the opening at the location was a full median opening allowing all possible movements. Accordingly some of the left turning vehicles made direct left turns while some other drivers selected indirect left turns. During this time field data was collected for a period of one week. The median was then converted into a directional median opening by using temporary physical barriers and field data were collected again in the same way for another week. This time period provided the information on the after period in which case the left-turning drivers coming from the driveway were expected to do it indirectly. Data Operational and safety evaluation of the two types of median openings were conducted by using the field data collected during before and after periods. All traffic movement data required for the operational analysis and conflict data required for the safety analysis were collected by using 3video cameras covering the whole intersection and the weaving distance. Cameras were installed at a height of approximately 15 ft above the ground so that the vehicles traveling on middle or outer lanes would not cover the vehicles traveling in the inner lane. Two reasons supported the idea of using video cameras. First, it would be extremely difficult to manually record traffic conflicts, exact timings and traffic volumes at the same time because observers would have to track and identify the maneuver of each vehicle when it departed from the side street. Second, on-site observations could not easily be verified, which is of especial consideration when observers have to record several different types of conflicts (3). Traffic volumes on the arterial were recorded by using an automatic data recorder ADR-1000 from Peek Traffic™ installed on the pavement. Right turn, direct left-turn, left-turn in, and right turn plus U-Turn maneuver volumes were obtained reviewing the videotapes at the laboratory. All cameras and the counter were synchronized before the start of the data collection, which eliminated tedious matching of vehicles, especially when 3 or 4 cameras were involved simultaneously.
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Data were collected during weekdays under normal traffic conditions, good weather, and dry pavement conditions. Weekdays in this study were considered from Monday through Thursday where normal conditions were expected to prevail. Also, data was collected during peak and non-peak periods. The morning peak hour was considered between 7:00 AM and 9:00 AM, and the afternoon peak period was considered between 4:00 p.m. and 6:00 p.m. Analysis Traffic and conflict data obtained through the field data collection were analyzed to compare operational and safety effects during before and after time periods, which represented full and directional median openings, respectively. Operational Analysis Traffic operational comparison of the two types of median openings was carried out by using two parameters, total delay and total travel time of left turning vehicles from the driveway (2). Total waiting delay of the direct left turning vehicles consisted of delay at the driveway and the delay at the median opening. Total waiting delay for indirect left turns at the directional median opening consisted of delay at the driveway and delay at the downstream median opening where the U-turn was made. Total travel time was obtained by adding the total running time to total delay. Since the before period could accommodate both direct and indirect left turns, Weighted average travel delay and weighted average travel time were used for comparison purposes. Safety Analysis Conflict data were analyzed in this study to evaluate the safety impacts of the two types of median openings (3). A total of 371 conflicts were recorded before the full median opening was converted to a directional median opening, and a total of 327 conflicts after the improvement. Hourly number of conflicts, daily number of conflicts and conflict rates (conflicts per thousand involved vehicles) were estimated for comparison purposes. RESULTS Operational Analysis Results of the operational analysis using weighted average delay and weighted average travel time are given in Figures 2 and 3, respectively. It was found that the differences in delay during before and after periods were statistically significant for both peak and non-peak periods. As for the travel time, even though there is a slight reduction during the after period the difference was not statistically significant. Accordingly, even if the directional median opening yields lesser delay for left-turning vehicles there is no change in the total travel time since it requires longer distance to be traveled.
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70
WATD(sec. per veh.)
60
Before
60.18
After
51.20
50 39.92
40
31.33
30 20 10 0 Peak
Time Period
Non-Peak
FIGURE 2. Before and After Comparison of Weighted Average Travel Delay 80 70
67.45
66.00
Before
After
WATT ( sec./veh.)
60 46.12
50
45.33
40 30 20 10 0 Peak
Non-Peak Time Period
FIGURE 3. Before and After Comparison of Weighted Average Travel Time Safety Analysis Results of the conflict analysis to evaluate the safety effects of the two median opening types are given in Tables 1 and 2, by considering total number of conflicts per hour and conflicts per thousand involved vehicles. It was seen that the number of conflicts per hour and conflicts per thousand involved vehicles were both significantly reduced due to the change in the median opening type to directional. Percentage reductions in conflicts per hour and conflicts per thousand involved vehicles were 49.9 percent and 46.3 percent, respectively. In a detailed analysis of severity of reported conflicts (that is not described in this paper), it was found that the severity of conflicts was also less for the after period.
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TABLE 1. Comparison of Total Number of Conflicts per Hour During Before and After Time Periods Number of Conflicts/Hour During the Before Period Due to Due to Total DLT RTUT 23.92
3.20
27.12
19.00
4.25
23.25
21.46
3.72
25.18
Reduction %
Number of Conflicts/Hour During the After Period Due to Due to Total DLT RTUT Peak Period 0 13.14 13.14 Non-peak Period 0 12.06 12.06 Total Average 0 12.60 12.60
(Before – After) x 100 Before 51.5% 48.1% 49.9%
TABLE 2. Average Number of Conflicts per Thousand Involved Vehicles Time 07:00–08:00 08:00–09:00 09:00–10:00 10:00–11:00 11:00–12:00 12:00–13:00 13:00–14:00 14:00–15:00 15:00–16:00 16:00–17:00 17:00–18:00 Average
Conflicts per Thousand Involved Vehicles Before After 118.71 97.94 54.68 55.31 51.00 73.64 62.43 55.62 72.22 77.54 65.59 71.33 Average Reduction = 46.3%
26.82 27.85 37.08 53.63 61.96 27.48 32.82 44.86 34.49 46.28 28.04 38.30
CONCLUSIONS AND RECOMMENDATIONS This case study proved the fact that access management treatments could be used effectively to improve traffic operations and/or safety. Even though studying only one location does not statistically provide sufficient evidence, this case study was conducted as a supplement to a bigger project that studied the subject. Using the same site in the form of a before and after study accounted for the argument that often arises of differences in site characteristics when multiple sites were used. By converting a full median opening into a directional median opening, the weighted average delay experienced by left turning vehicles was significantly reduced even though the reduction in total travel time was unaffected. However, safety effects of the conversion was highly significant where the conflicts per hour and conflicts per thousand involved vehicles were reduced 49.9 percent and 46.3 percent, respectively. Accordingly the conversion was highly successful under the prevailing conditions.
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ACKNOWLEDGMENTS This case study was completed as a part of a much larger project, which was funded by the Florida Department of Transportation. REFERENCES 1. National Highway Institute. Access Management, Location and Design, Participant Notebook. NHI Course No. 133078. Federal Highway Administration, U.S. Department of Transportation,
Washington, D.C., 2000. 2. Lu, J., S. Dissanayake, H. Zhou, X.K. Yang, and K. Williams. Operational Evaluation of Right Turns Followed by U-Turns as an Alternative to Direct Left Turns. Department of Civil and Environmental Engineering, University of South Florida, Tampa, Florida, October 2001. 3. Lu, J., S. Dissanayake, N. Castillo, and K. Williams. Safety Evaluation of Right Turns Followed by U-Turns as an Alternative to Direct Left Turns: Conflict Analysis. Department of Civil and Environmental Engineering, University of South Florida, Tampa, Florida, October 2001.
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