National Academies Press: OpenBook

Safety of U-Turns at Unsignalized Median Openings (2004)

Chapter: Chapter 6 - Findings

« Previous: Chapter 5 - Data Collection and Analysis
Page 68
Suggested Citation:"Chapter 6 - Findings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Chapter 6 - Findings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Chapter 6 - Findings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Chapter 6 - Findings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Chapter 6 - Findings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Chapter 6 - Findings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
×
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Suggested Citation:"Chapter 6 - Findings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Chapter 6 - Findings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
×
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Suggested Citation:"Chapter 6 - Findings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Chapter 6 - Findings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
×
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Suggested Citation:"Chapter 6 - Findings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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68 CHAPTER 6 FINDINGS This chapter presents the findings of the research. The issues addressed in this chapter are accident and field data, median opening accident frequencies, median opening accident rates, median opening traffic conflicts, comparison of acci- dent rates and conflict rates, and combinations of median opening types. ACCIDENT AND FIELD DATA Table 29 presents the number of unsignalized median open- ing sites in each of the following categories: • Median openings in the original catalog, • Median openings with accident data available, • Median openings where field data (either videotape stud- ies or supplementary traffic counts) were collected, and • Median openings with both accident and field data. The median openings in the original catalog are all of the unsignalized median openings that were visited and docu- mented in the field. They represent all of the original median openings available for possible inclusion in the analyses. However, accident data were not available for all of the median openings. In one state, accident data were available only for intersection locations (i.e., locations with a cross street). Thus, no accident data were available for midblock locations (Types 1a, 1b, 2b, and 2c). In other states, some of the corridors orig- inally recommended by the participating highway agency— and which were included in the catalog of median opening types—were not maintained by the highway agency and, therefore, accident data were not available for these locations either. In Table 29, the columns under “Median openings with field data” illustrate the number of median openings where videotape studies were conducted (“Primary”) and where 15-min counts were made (“Supplementary”). The last column represents the median openings with both accident and field data and, given that accident and traffic volume data are needed to conduct meaningful accident analyses, the median openings that were included in the analyses. Thus, the proj- ect database includes 115 unsignalized median opening sites with both accident and field data. Table 30 summarizes traffic volume data by median open- ing type. The average major-road ADT is fairly consistent across all median opening types, with the exceptions of Type 1a (i.e., conventional midblock median opening without left- turn lanes) and Type 3a (i.e., conventional median opening without left-turn lanes at three-leg intersection), which each have about one-half the average ADT on the major road. The median opening volumes represent all U-turns and left turns that go through the median opening, including left turns from the minor road; these volumes are based on counts made during the field studies. The midblock median openings (Types 1a, 2b, and 2c) have no left-turn volumes given that, by design, they accommodate only U-turn traffic. The median opening volumes constitute a small percentage of the major road volumes, ranging from 0.0 to 4.2 percent. MEDIAN OPENING ACCIDENT FREQUENCIES Tables 31 through 33 present frequencies of total acci- dents, fatal and injury accidents, and PDO accidents, respec- tively, by median opening type. U-turn and left-turn accident frequencies are presented separately as well as in combina- tion. These frequencies should not be considered conclusions about the relative safety of various designs because they do not consider traffic volumes (i.e., median types showing more accidents may also have had higher traffic volumes). Table 31 demonstrates that accidents related to U-turn and left-turn maneuvers at unsignalized median openings occur very infrequently. The 103 median openings in urban arter- ial corridors experienced an average of 0.41 U-turn plus left- turn accidents per median opening per year. The 12 median openings in rural arterial corridors experienced an average of 0.20 accidents per median opening per year. Overall, at these median openings, U-turns represent 58 percent of the median opening movements and left turns represent 42 percent of the median opening movements. Based on these limited accident frequencies, there is no indication that U-turns at unsignal- ized median openings constitute a major safety concern. Table 34 presents the distribution of collision types by median opening type. Collision types are as follows: • Major-road angle collision—collision between a major- road through vehicle and a vehicle turning from the median opening onto the major road (i.e., either a left- turning vehicle or a U-turn vehicle)

69 • Major-road rear-end collision—collision between a vehicle making either a U-turn or left-turn from the major road into the median opening and a following major- road through vehicle • Cross-street collision—any collision involving a vehi- cle from the cross street • Other or unknown collision type—collision where the intended vehicle movements are unknown MEDIAN OPENING ACCIDENT RATES Table 35 presents total median opening accident rate by median opening type. The median opening accident rate con- sists of the number of accidents involving either a U-turn or left turn through the median opening per million vehicles turning through the median opening. For urban arterial corridors, median opening accident rates are substantially lower for midblock median openings than for median openings at three-leg and four-leg intersections. For example, the accident rate per million median opening movements (i.e., U-turn plus left-turn maneuvers) at a direc- tional midblock median opening is typically only about 14 percent of the median opening accident rate for a directional median opening at a three-leg intersection. At conventional three-leg median openings, the average median opening accident rate at median openings with two left-turn lanes (Type 3c) is substantially higher than the average median Median openings with field data Median opening type Median openings in original catalog Median openings with accident data available Primary Supplementary Total Median openings with both accident and field data URBAN ARTERIAL CORRIDORS Midblock 1a 12 10 0 7 7 7 1b 23 1 0 1 1 0 2b 149 145 8 12 20 20 2c 11 10 5 8 13 10 Three-leg 3a 44 34 1 10 11 11 3b 183 99 5 25 30 19 3c 4 4 0 2 2 2 4a 37 20 4 7 11 4 4b 1 0 0 0 0 0 Four-leg 5a 43 41 2 6 8 8 5b 164 106 4 26 30 17 6a 14 10 1 5 6 5 Subtotal 685 480 30 109 139 103 RURAL ARTERIAL CORRIDORS Midblock 1a 29 27 1 6 7 7 1b 6 6 0 0 0 0 2a 1 0 0 0 0 0 2c 1 0 0 0 0 0 Three-leg 3a 63 49 0 4 4 4 3b 24 22 0 0 0 0 3c 21 21 0 0 0 0 4a 2 1 0 0 0 0 Four-leg 5a 55 43 1 0 1 1 5b 20 19 0 0 0 0 5c 2 2 0 0 0 0 6a 1 1 0 0 0 0 Subtotal 225 191 2 10 12 12 TOTAL 910 671 32 119 151 115 TABLE 29 Number of unsignalized median opening sites

70 Turning volume at median openings (veh/day) (percentage of major road ADT) (percentage of total median opening ADT) Median opening type Number of median openings Average two-way major-road ADTa (veh/day) U-turn Left turn Total U-turn Left turn Total U-turn Left turn URBAN ARTERIAL CORRIDORS Midblock 1a 7 13,161 2 0 2 0.01 0.00 0.01 100.00 0.00 2b 20 33,495 700 0 700 2.09 0.00 2.09 100.00 0.00 2c 10 30,231 977 0 977 3.23 0.00 3.23 100.00 0.00 Three-leg 3a 11 14,446 11 95 106 0.08 0.66 0.74 10.38 89.62 3b 19 32,760 248 187 435 0.76 0.57 1.33 57.01 42.99 3c 2 42,361 79 307 386 0.19 0.72 0.91 20.47 79.53 4a 4 31,366 138 680 818 0.44 2.17 2.61 16.87 83.13 Four-leg 5a 8 34,324 259 622 881 0.75 1.81 2.56 29.40 70.60 5b 17 40,096 285 533 818 0.71 1.33 2.04 34.84 65.16 6a 5 38,476 552 1,065 1,617 1.43 2.77 4.20 34.14 65.86 RURAL ARTERIAL CORRIDORS Midblock 1a 7 21,309 88 0 88 0.41 0.00 0.41 100.00 0.00 Three-leg 3a 4 27,448 374 404 778 1.36 1.47 2.83 48.07 51.93 Four-leg 5a 1 28,126 346 555 901 1.23 1.97 3.20 38.40 61.60 a Major-road and median-opening volumes for year 2002. TABLE 30 Turning volumes at median openings Median opening accident frequencya (for entire study period) Median opening accident frequency (per median opening per year) Median opening type Number of median openings U-turn Left-turn Total U-turn Left-turn Total URBAN ARTERIAL CORRIDORS Midblock 1a 7 1 0 1 0.029 0.000 0.029 2b 20 1 3 4 0.010 0.030 0.040 2c 10 2 3 5 0.040 0.060 0.100 Three-leg 3a 11 1 8 9 0.018 0.145 0.164 3b 19 5 27 32 0.055 0.297 0.352 3c 2 0 10 10 0.000 1.000 1.000 4a 4 0 7 7 0.000 0.304 0.304 Four-leg 5a 8 0 26 26 0.000 0.650 0.650 5b 17 5 71 76 0.056 0.798 0.854 6a 5 3 39 42 0.100 1.300 1.400 RURAL ARTERIAL CORRIDORS Midblock 1a 7 0 3 3 0.000 0.088 0.088 Three-leg 3a 4 0 4 4 0.000 0.235 0.235 Four-leg 5a 1 2 2 4 0.500 0.500 1.000 a The study period was generally five years in duration. However, only four years of accident data were available for sites in New Jersey, and six years of accident data were available for sites in New York. TABLE 31 Median opening accident frequency by median opening type

71 opening accident rate at median openings with either no left- turn lane or only one left-turn lane, but the data reflect only two median openings of Type 3c. Furthermore, the data showed considerable state-to-state variation that could not be accounted for with a database of this size. Comparing median openings at three-leg intersections, average median opening accident rates for directional three-leg median openings are about 48 percent lower than the accident rates for conventional three-leg median openings. Comparing median openings at four-leg intersec- tions, average median opening accident rates for directional four-leg median openings are about 15 percent lower than for conventional four-leg intersections. For rural arterial corridors, the average median opening accident rate is lower for median openings at three-leg inter- sections than for median openings at four-leg intersections. However, the sample size of median openings and median- opening-related accidents for rural arterial corridors is so small that no firm conclusions can be drawn. Comparable results for fatal-and-injury and property- damage-only accident rates at median openings are presented in Tables 36 and 37. MEDIAN OPENING CONFLICT RATES An analysis of traffic conflicts was conducted and is described in Chapter 5. The videotape recordings made dur- ing the field data collection effort were reviewed to document how drivers behave in making U-turns and left turns through unsignalized median openings. Three basic conflict types were observed and analyzed: • Conflict Type A: A vehicle turning from the major road into the median opening causes the following vehicle on the major road to brake. • Conflict Type B: Conflict between two or more vehicles within the median opening. • Conflict Type C: A vehicle turning from the median opening onto the major road causes the vehicle on the major road to brake. Table 38 summarizes traffic conflicts by median opening type. Some observations from the table are as follows: Median opening accident frequencya (for entire study period) Median opening accident frequency (per median opening per year) Median opening type Number of median openings U-turn Left-turn Total U-turn Left-turn Total URBAN ARTERIAL CORRIDORS Midblock 1a 7 1 0 1 0.029 0.000 0.029 2b 20 1 2 3 0.010 0.020 0.030 2c 10 1 0 1 0.020 0.000 0.020 Three-leg 3a 11 1 3 4 0.018 0.055 0.073 3b 19 0 13 13 0.000 0.143 0.143 3c 2 0 6 6 0.000 0.600 0.600 4a 4 0 4 4 0.000 0.174 0.174 Four-leg 5a 8 0 10 10 0.000 0.250 0.250 5b 17 0 24 24 0.000 0.270 0.270 6a 5 2 25 27 0.067 0.833 0.900 RURAL ARTERIAL CORRIDORS Midblock 1a 7 0 2 2 0.000 0.059 0.059 Three-leg 3a 4 0 1 1 0.000 0.059 0.059 Four-leg 5a 1 0 0 0 0.000 0.000 0.000 a The study period was generally five years in duration. However, only four years of accident data were available for sites in New Jersey, and six years of accident data were available for sites in New York. TABLE 32 Fatal-and-injury median opening accident frequency by median opening type

• Most traffic conflicts at midblock median openings and median openings at three-leg intersections, particularly directional median openings, were related to the turning vehicle merging onto the major road from the median opening (Conflict Type C). • At median openings without left-turn lanes at four-leg intersections, most of the traffic conflicts were related to vehicles turning from the major road into the median opening (Conflict Type A). • At directional median openings (Types 2b, 2c, 4a, and 6a), the greatest problem appears to be related to vehi- cles merging onto the major road from the median open- ing, as evidenced by the large number of Type C traffic conflicts. COMPARISON OF MEDIAN OPENING ACCIDENT AND CONFLICT RATES Table 39 presents median opening accident and conflict rates by geometry type and number of intersection legs. All of the median openings are urban. Relative safety compar- isons that can be made from the table include the following: 72 • Median opening accident rates are substantially lower for midblock median openings than for median open- ings at three- and four-leg intersections. For example, the median opening accident rate at a directional mid- block median opening is typically only about 14 percent of the median opening accident rate for a directional median opening at a three-leg intersection. • Median opening accident rates are slightly lower at con- ventional three-leg median openings than at conven- tional four-leg median openings. Median opening con- flict rates at conventional three-leg median openings are almost half the conflict rates at conventional four-leg median openings. • Median opening accident rates at directional three-leg median openings are about 48 percent lower than the accident rates for conventional three-leg openings. In contrast, median opening conflict rates at directional three-leg median openings are substantially higher than the conflict rates for conventional three-leg openings. • Median opening accident rates at directional four-leg median openings are about 15 percent lower than for conventional four-leg intersections. In contrast, median opening conflict rates at directional four-leg median Median opening accident frequencya (for entire study period) Median opening accident frequency (per median opening per year) Median opening type Number of median openings U-turn Left-turn Total U-turn Left-turn Total URBAN ARTERIAL CORRIDORS Midblock 1a 7 0 0 0 0.000 0.000 0.000 2b 20 0 1 1 0.000 0.010 0.010 2c 10 1 3 4 0.020 0.060 0.080 Three-leg 3a 11 0 5 5 0.000 0.091 0.091 3b 19 5 14 19 0.055 0.154 0.209 3c 2 0 4 4 0.000 0.400 0.400 4a 4 0 3 3 0.000 0.130 0.130 Four-leg 5a 8 0 16 16 0.000 0.400 0.400 5b 17 5 47 52 0.056 0.528 0.584 6a 5 1 14 15 0.033 0.467 0.500 RURAL ARTERIAL CORRIDORS Midblock 1a 7 0 1 1 0.000 0.029 0.029 Three-leg 3a 4 0 3 3 0.000 0.176 0.176 Four-leg 5a 1 2 2 4 0.500 0.500 1.000 a The study period was generally five years in duration. However, only four years of accident data were available for sites in New Jersey, and six years of accident data were available for sites in New York. TABLE 33 Property-damage-only median opening accident frequency by median opening type

73 Accident frequency by collision typea (for entire study period) Percentage of accident frequency by collision type Median opening type Number of median openings Major- road angle Major- road rear end Cross street Other or unknown Total Major road angle Major-road rear end Cross street Other or unknown URBAN ARTERIAL CORRIDORS Midblock 1a 7 1 0 0 0 1 100.0 0.0 0.0 0.0 2b 20 2 2 0 0 4 50.0 50.0 0.0 0.0 2c 10 1 3 0 1 5 20.0 60.0 0.0 20.0 Three-leg 3a 11 0 4 4 1 9 0.0 44.4 44.4 11.1 3b 19 8 5 18 1 32 25.0 15.6 56.3 3.1 3c 2 9 1 0 0 10 90.0 10.0 0.0 0.0 4a 4 7 0 0 0 7 100.0 0.0 0.0 0.0 Four-leg 5a 8 16 8 2 0 26 61.5 30.8 7.7 0.0 5b 17 28 33 12 3 76 36.8 43.4 15.8 3.9 6a 5 35 2 3 2 42 83.3 4.8 7.1 4.8 RURAL ARTERIAL CORRIDORS Midblock 1a 7 0 2 1 0 3 0.0 66.7 33.3 0.0 Three-leg 3a 4 0 3 1 0 4 0.0 75.0 25.0 0.0 Four-leg 5a 1 1 2 1 0 4 25.0 50.0 25.0 0.0 a The duration of the study period was generally five years. However, only four years of accident and exposure data were available for sites in New Jersey, and six years of accident and exposure data were available for sites in New York. TABLE 34 Collision type distribution for median opening turning accidents Median opening type Number of median openings Total median opening accident frequencya (for entire study period) Median opening movements (106 turns during entire study period) Median opening accident rate (accidents per 106 turning vehicles) URBAN ARTERIAL CORRIDORS Midblock 1a 7 1 –b –b 2b 20 4 17.20 0.23 2c 10 5 13.42 0.37 Three-leg 3a 11 9 2.23 4.04 3b 19 32 13.04 2.46 3c 2 10 1.20 8.35 4a 4 7 4.87 1.44 Four-leg 5a 8 26 11.16 2.33 5b 17 76 22.77 3.34 6a 5 42 16.36 2.57 RURAL ARTERIAL CORRIDORS Midblock 1a 7 3 0.96 3.13 Three-leg 3a 4 4 4.65 0.86 Four-leg 5a 1 4 1.41 2.84 a The duration of the study period was generally five years. However, only four years of accident and exposure data were available for sites in New Jersey, and six years of accident and exposure data were available for sites in New York. b Data too limited to be meaningful. TABLE 35 Total median opening accident rate by median opening type

openings are more than twice the conflict rates at con- ventional four-leg median openings. COMBINATIONS OF MEDIAN OPENINGS Comparisons between the safety of conventional and direc- tional median openings and between midblock median open- ings and median openings at three- and four-leg intersections are more reasonable when the number of movements allowed through each median opening type is included in the com- parison. For example, median opening accident rates are sub- stantially lower for midblock median openings than for median openings at three- and four-leg intersections. This is not sur- prising given that midblock median openings do not have cross-street traffic turning through the median opening, thus creating additional points of conflict. Similarly, median open- ing accident rates at directional three-leg median openings are about 48 percent lower than the accident rates for con- ventional three-leg openings. Directional three-leg median openings (Type 4a) only accommodate left turns and U-turns from the major road and do not permit left turns from the 74 cross street. Therefore, to conduct a fair comparison of con- ventional and directional median openings at three-leg inter- sections, combinations of median openings should be com- pared. For example, a directional three-leg median opening in combination with a directional midblock median opening, as presented in Figure 47, is better compared with a conven- tional three-leg opening (Type 3b) because the same turning movements are accommodated in each situation. This section presents several typical combinations of median opening designs that can be used along an arterial to accom- modate the same turning movements allowed at an individ- ual median opening. In each case, a safety comparison is made between the individual median opening and the combination of median openings. Comparison of Conventional and Directional Median Openings at Three-Leg Intersections A conventional median opening at a three-leg intersection (Type 3a, 3b, 3c, or 3d) accommodates all turning movements from the major road and cross street. A directional median Median opening type Number of median openings Total median opening accident frequencya (for entire study period) Median opening movements (106 turns during entire study period) Median opening accident rate (accidents per 106 turning vehicles) URBAN ARTERIAL CORRIDORS Midblock 1a 7 1 –b –b 2b 20 3 17.20 0.17 2c 10 1 13.42 0.08 Three-leg 3a 11 4 2.23 1.80 3b 19 13 13.04 1.00 3c 2 6 1.20 5.01 4a 4 4 4.87 0.82 Four-leg 5a 8 10 11.16 0.90 5b 17 24 22.77 1.05 6a 5 27 16.36 1.65 RURAL ARTERIAL CORRIDORS Midblock 1a 7 2 0.96 2.09 Three-leg 3a 4 1 4.65 0.22 Four-leg 5a 1 0 1.41 0.00 a The duration of the study period was generally five years. However, only four years of accident and exposure data were available for sites in New Jersey, and six years of accident and exposure data were available for sites in New York. b Data too limited to be meaningful. TABLE 36 Fatal-and-injury median opening accident rate by median opening type

75 Median opening type Number of median openings Total median opening accident frequencya (for entire study period) Median opening movements (106 turns during entire study period) Median opening accident rate (accidents per 106 turning vehicles) URBAN ARTERIAL CORRIDORS Midblock 1a 7 0 –b –b 2b 20 1 17.20 0.06 2c 10 4 13.42 0.30 Three-leg 3a 11 5 2.23 2.24 3b 19 19 13.04 1.46 3c 2 4 1.20 3.34 4a 4 3 4.87 0.62 Four-leg 5a 8 16 11.16 1.43 5b 17 52 22.77 2.28 6a 5 15 16.36 0.92 RURAL ARTERIAL CORRIDORS Midblock 1a 7 1 0.96 1.04 Three-leg 3a 4 3 4.65 0.65 Four-leg 5a 1 4 1.41 2.84 a The duration of the study period was generally five years. However, only four years of accident and exposure data were available for sites in New Jersey, and six years of accident and exposure data were available for sites in New York. b Data too limited to be meaningful. TABLE 37 Property-damage-only median opening accident rate by median opening type Number of conflicts by conflict type Type Number of conflicts No. of conflicts per 103 major- road vehicles Aa Bb Cc Midblock 2b 6 2.9 1 0 5 2c 36 20.6 0 1 35 Three-leg 3b 12 6.4 4 2 6 4a 43 34.1 6 0 37 Four-leg 5a 19 12.8 17 2 0 5b 31 10.2 7 5 19 6a 20 27.6 1 0 19 a Conflict Type A: Vehicle turning from the major road into the median opening causes following vehicle on the major road to brake. b Conflict Type B: Conflict between two or more vehicles within the median opening. c Conflict Type C: Vehicle turning from the median opening onto the major road causes vehicle on the major road to brake. TABLE 38 Summary of traffic conflicts by median opening type at urban sites

opening at a three-leg intersection accommodates only left turns from the major road to the cross street (Type 4a) or left turns from the cross street to the major road (Type 4b). There- fore, a directional median opening at a three-leg intersection may be used in combination with a directional midblock median opening (Type 2a or 2b) to indirectly accommodate all turning movements. Figure 47 presents a conventional median opening at a three-leg intersection (Type 3b) and a combination of a directional median opening at a three-leg intersection (Type 4a) with a directional midblock median opening (Type 2b). From Table 39, the median opening accident rate for a con- ventional median opening at a three-leg intersection (Type 3a + 3b) is 2.69 accidents per million turning vehicles. From Tables 35 and 39, the median opening accident rate for the combination of directional median openings is 1.63 accidents per million turning vehicles (1.40 for the Type 4a median opening plus 0.233 for the Type 2b median opening). Thus, the median opening accident rate for the combination of directional median openings is about 40 percent less than the accident rate for a conventional median opening at a three- leg intersection. Comparison of Conventional and Directional Median Openings at Four-Leg Intersections A conventional median opening at a four-leg intersection (Type 5a, 5b, or 5c) accommodates all turning movements from the major road and cross street as well as through move- ments on the cross street. A directional median opening at a four-leg intersection accommodates only left turns from the 76 major road to the cross street (Type 6a). Therefore, a direc- tional median opening at a four-leg intersection may be used in combination with two directional midblock median open- ings (Type 2a or 2b) to indirectly accommodate all turning movements. Figure 48 presents a conventional median open- ing at a four-leg intersection (Type 5b) and a combination of a directional median opening at a four-leg intersection (Type 6a) with two directional midblock median openings (Type 2b). From Table 39, the median opening accident rate for a conventional median opening at a four-leg intersection (Type 5a + 5b) is 3.01 accidents per million turning vehicles. From Tables 34 and 39, the median opening accident rate for the combination of directional median openings is 3.03 accidents per million turning vehicles (2.57 for the Type 6a median opening plus 0.46 for two Type 2b median openings), which is about equal to the median opening accident rate for the conventional median opening. Comparison of Conventional and Directional Median Openings at Signalized Four-Leg Intersections Although the focus of the research was on unsignalized median openings, most of the directional midblock median openings in the study were sites in Michigan, where they are used in combination with signalized intersections. At these locations, left turns are prohibited from the major and minor road at the signalized intersection. From the minor road, left- turning traffic turns right onto the divided road and then makes a U-turn at the directional midblock median opening. Traffic turning left from the major road must first travel Median opening type Number of median openings Number of accidents Median opening movements (106 turns during entire study period) Median opening accident rate (accidents per 106 turning vehicles) Conflict rate (conflicts per 103 major-road vehicles) Directional midblock 30 9 30.62 0.29 11.0 (2b + 2c) Conventional 3-leg 30 41 15.27 2.69 6.4a (3a + 3b) Directional 3-leg 4 7 4.87 1.40 34.1 (4a) Conventional 4-leg 25 102 33.93 3.01 11.0 (5a + 5b) Directional 4-leg 5 42 16.36 2.57 27.6 (6a) a Conflict data collected at Type 3b median openings only. TABLE 39 Median opening accident and conflict rates by geometry type and number of intersection legs at urban sites

77 Figure 47. Conventional and directional median openings at three-leg intersection. a) Conventional Median Opening at Three-Leg Intersection (Type 3b) b) Directional Median Opening For Left Turns From Major Road at Three-Leg Intersection With Directional Midblock Median Opening (Type 4a + Type 2b) a) Conventional Median Opening at Four-Leg Intersection (Type 5b) b) Directional Median Opening Four-Leg Intersection With Two Directional Midblock Median Openings (Type 2b + Type 6a + Type 2b) Figure 48. Conventional and directional median openings at four-leg intersection. through the intersection and then make a U-turn at the direc- tional midblock median opening, followed by a right turn onto the minor road. Figure 49 illustrates a conventional signalized four-leg intersection and a combination of directional midblock median openings with a signalized intersection where turns are pro- hibited. Without a formal safety prediction model for signal- ized intersections on divided highways that is sensitive to turning volumes, no formal comparison of the relative safety of the two scenarios (conventional and directional median openings at signalized four-leg intersections) can be made. However, given that the median opening accident rate for directional midblock median openings is so low, it is likely that the safety performance of the directional median opening combination, as used in Michigan, is as good as or better than the safety performance of the conventional median opening.

78 Figure 49. Median openings at signalized four-leg intersection. Signal Traffic Traffic Signal a) Conventional Median Opening at Signalized Four-Leg Intersection b) Signalized Four-Leg Intersection With No Turns Permitted and Two Directional Midblock Median Openings

Next: Chapter 7 - Conclusions and Recommendations »
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 524: Safety of U-Turns at Unsignalized Median Openings includes recommended guidelines for locating and designing unsignalized median openings, and a methodology for comparing the relative safety performance of different designs.

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