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26 Description Techniques in this group involve configuring the design of unsignalized median openings, including providing left-turn channelization, adjusting the median width, and installing left- turn acceleration lanes. These techniques can be considered wherever a non-traversable median exists to improve roadway operations and safety where openings in the median are provided. See Chapter 2 for information relating to medians in general. Tables 14 through 16 follow. Quantitative Analysis Methods Motor Vehicle Operations Methods in Chapter 20 in the HCM6 (2) can be used to compare intersection operations with and without two-stage minor street left-turn operation. The operation of an intersection provid- ing steady flow in one direction can be evaluated using the HCM6 by setting the through volume for the steady-flow direction to zero. Motor Vehicle Safety The HSM (4) provides the following crash modification factors (CMFs) for multiple-vehicle crashes related to widening the intersection median width in 3-feet increments: ⢠Rural, four-leg unsignalized: 0.96 (all severities), 0.96 (injury) ⢠Urban and suburban, four-leg unsignalized: 1.06 (all severities), 1.05 (injury) C H A P T E R 5 Unsignalized Median Openings Source: Photograph provided by Google Earth.
Increase median width to store left turn egress vehicles. â â â Â Â â â Â Â Â Â Â Â Â Channelize median to control merge of left turn egress vehicles. Â Â Â Â Â â Â Â Â Â Develop left turn acceleration lane. â Â â Â Â â Â Â â â Provide full access with steady flow in one direction of arterial. â â â Â Â Â â â Â Â Â Channelize left turns to keep vehicles from returning to through lanes. Â Â Â Â Â â Â Â Â Â Â Channelize left turns across wide medians to improve offset. Â â â Â Â â â Â Â Â Â Â Increase effective approach width of right-angle median crossovers. Â Â Â Â Â Â Â Â Â Â Access Management Technique Performance Trends and Documented Performance Relationships Operations Safety Note: â = unchanged performance. Table 14. Multimodal operations and safety performance summary. Mode Operations Safety Increasing the median width to provide storage space for minor street left-turning vehicles allows those vehicles to complete the maneuver in two stages, increasing the left-turn capacity and reducing delay (1, 2). Left-turn acceleration lanes substantially reduce delay for the second stage of a two-stage left turn (3). Providing steady flow for one direction by using channelizing islands may eliminate the need for signalizing a three-leg median opening. Increasing the median width slightly reduces the crash rate at rural intersections (3, 4) but slightly increases it at urban intersections (4). Increasing raised median width decreases the crash rate along urban and suburban arterials (5). Offset left-turn lanes (3) and very wide medians (4) at rural intersections may increase the potential for wrong-way movements. Left- turn deceleration lane channelization prevents unexpected maneuvers back into the through lane (6) but may cause drivers to begin their acceleration sooner, increasing the speed differential between left-turning vehicles and through vehicles (3). Left-turn acceleration lane channelization reduces the speed differential between minor street left turns and through vehicles, improving safety for minor street left turns, but the reduction in the left-turn lane offset may decrease safety for major street left turns (3). Increasing the median width to provide a pedestrian refuge allows two-stage pedestrian crossings, reducing pedestrian delay (1, 2). Sufficiently wide channelizing islands used to create offset left-turn lanes can also act as pedestrian refuges (1). Sufficiently wide pedestrian refuges and islands provide pedestrian refuge (1, 4). Increasing raised median and two-way left-turn lane width decreases the pedestrian crash rate along urban and suburban arterials (5). Steady-flow designs prevent the potential for establishing a pedestrian crossing. No direct effect. See Chapter 1. Steady-flow designs require bicyclists to make a left turn as a vehicle (i.e., no option to cross as a pedestrian). Similar to motor vehicles. No documented effect beyond that generally observed for motor vehicle traffic. Similar to motor vehicles. Left-turn acceleration lanes have been installed specifically in situations where insufficient median width exists to store trucks (3). Table 15. General trends associated with improvements to unsignalized median openings.
28 Guide for the Analysis of Multimodal Corridor Access Management ⢠Urban and suburban, three-leg unsignalized: 1.03 (all severities) ⢠Urban and suburban, four-leg signalized: 1.03 (all severities), 1.03 (injury) The HSM also provides a CMF of 0.73 for providing a channelized left-turn lane for the major roadway at a rural 3-leg intersection on a 2-lane highway (4). NCHRP Report 650 (3) summarized the results of a limited number of previous studies on the safety of left-turn acceleration lanes. The general trend in these studies indicated improved overall safety, but potential biases in the study designs (e.g., regression to the mean) prevent making definitive conclusions. NCHRP Report 650 (3) also summarized the results of North Carolina studies of offset left- turn lanes, which found a reduction in left-turn-leaving crashes but an increase in rear-end crashes. Due to potential biases in the study designs (e.g., other simultaneous improvements, changes in traffic volumes, regression to the mean), no definitive conclusions can be drawn. Table 16 gives crash rate ratios for various speed differentials on rural highways, relative to no speed differential. For example, the crash rate with a 10-mph speed differential is twice the crash rate with no speed differential (7, 8). The vehicleâvehicle crash prediction model developed by Bowman et al. (5) for urban and suburban arterials with raised medians indicates that the crash rate increases as the number of median crossovers increases. The crash rate decreases with increasing raised median width. The appendix provides more details about this model. Pedestrian Operations Chapter 20 in the HCM6 can be used to compare pedestrian delay with and without a two-stage pedestrian crossing (2). This delay can also be used in Chapter 18 in the HCM6 to determine the change, if any, in pedestrian LOS. Pedestrian Safety The vehicleâpedestrian crash prediction models developed by Bowman et al. (5) for urban and suburban arterials with raised medians and two-way left-turn lanes indicated that the crash rate decreased with increasing raised median width and when the two-way left-turn lane width increased from 12 feet to 14 feet. The appendix provides more details about these models. Additional Information ⢠Chapters 1 and 9 in this guide. ⢠Access Management Manual, Second ed.: Section 17.3. Speed Differential (mph) Crash Rate Ratio Relative to no Speed Differential 0 1.0 â10 2.0 â20 6.5 â30 45 â35 180 Source: Solomon (7), Stover and Koepke (8). Table 16. Crash rate ratios for various speed differentials on rural highways.
Unsignalized Median Openings 29 ⢠Access Management Application Guidelines: Chapter 15, Median Applications and Design. ⢠Median Handbook: Chapter 2, Important Concepts of Medians and Median Openings Placement (Florida DOT). References 1. Williams, K. M., V. G. Stover, K. K. Dixon, and P. Demosthenes. Access Management Manual, Second ed. Transportation Research Board of the National Academies, Washington, D.C., 2014. 2. Highway Capacity Manual: A Guide for Multimodal Mobility Analysis, 6th ed. Transportation Research Board, Washington, D.C., 2016. 3. Maze, T. H., J. L. Hochstein, R. R. Souleyrette, H. Preston, and R. Storm. NCHRP Report 650: Median Intersection Design for Rural High-Speed Divided Highways. Transportation Research Board of the National Academies, Washington, D.C., 2010. 4. Highway Safety Manual, 1st ed. American Association of State Highway and Transportation Officials, Washington, D.C., 2010. 5. Bowman, B., R. Vecellio, and J. Miao. Vehicle and Pedestrian Accident Models for Median Locations. Journal of Transportation Engineering, Vol. 121, No. 6, American Society of Civil Engineers, Washington, D.C., Nov.âDec. 1994, pp. 531â537. 6. Gluck, J., H. S. Levinson, and V. Stover. NCHRP Report 420: Impacts of Access Management Techniques. Transportation Research Board, Washington, D.C., 1999. 7. Solomon, D. H. Accidents on Main Rural Highways Related to Speed, Driver, and Vehicle. Bureau of Public Roads, Washington, D.C., 1964. 8. Stover, V. G., and F. J. Koepke, Transportation and Land Development, 2nd ed., Institute of Transportation Engineers, Washington, D.C., 2002.
