Cover Image

Not for Sale



View/Hide Left Panel
Click for next page ( 75


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 74
74 TABLE 36 Fatal-and-injury median opening accident rate by median opening type Total median opening accident Median opening a Median Number of frequency movements Median opening accident 6 6 opening median (for entire study (10 turns during rate (accidents per 10 type openings period) entire study period) turning vehicles) URBAN ARTERIAL CORRIDORS Midblock b b 1a 7 1 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. openings are more than twice the conflict rates at con- cross street. Therefore, to conduct a fair comparison of con- ventional four-leg median openings. 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 COMBINATIONS OF MEDIAN OPENINGS in combination with a directional midblock median opening, as presented in Figure 47, is better compared with a conven- Comparisons between the safety of conventional and direc- tional three-leg opening (Type 3b) because the same turning tional median openings and between midblock median open- movements are accommodated in each situation. ings and median openings at three- and four-leg intersections This section presents several typical combinations of median are more reasonable when the number of movements allowed opening designs that can be used along an arterial to accom- through each median opening type is included in the com- modate the same turning movements allowed at an individ- parison. For example, median opening accident rates are sub- ual median opening. In each case, a safety comparison is made stantially lower for midblock median openings than for median between the individual median opening and the combination openings at three- and four-leg intersections. This is not sur- of median openings. 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- Comparison of Conventional and Directional ing accident rates at directional three-leg median openings Median Openings at Three-Leg Intersections are about 48 percent lower than the accident rates for con- ventional three-leg openings. Directional three-leg median A conventional median opening at a three-leg intersection openings (Type 4a) only accommodate left turns and U-turns (Type 3a, 3b, 3c, or 3d) accommodates all turning movements from the major road and do not permit left turns from the from the major road and cross street. A directional median

OCR for page 74
75 TABLE 37 Property-damage-only median opening accident rate by median opening type Total median opening accident Median opening a Median Number of frequency movements Median opening accident 6 6 opening median (for entire study (10 turns during rate (accidents per 10 type openings period) entire study period) turning vehicles) URBAN ARTERIAL CORRIDORS Midblock b b 1a 7 0 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 38 Summary of traffic conflicts by median opening type at urban sites No. of conflicts Number of conflicts by conflict type 3 Number of per 10 major- a b c Type conflicts road vehicles A B C 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.

OCR for page 74
76 TABLE 39 Median opening accident and conflict rates by geometry type and number of intersection legs at urban sites Median opening movements Median opening Conflict rate 6 3 Number of (10 turns accident rate (conflicts per 10 6 Median median Number of during entire (accidents per 10 major-road opening type openings accidents study period) turning vehicles) vehicles) Directional midblock 30 9 30.62 0.29 11.0 (2b + 2c) Conventional a 3-leg 30 41 15.27 2.69 6.4 (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. opening at a three-leg intersection accommodates only left major road to the cross street (Type 6a). Therefore, a direc- turns from the major road to the cross street (Type 4a) or left tional median opening at a four-leg intersection may be used turns from the cross street to the major road (Type 4b). There- in combination with two directional midblock median open- fore, a directional median opening at a three-leg intersection ings (Type 2a or 2b) to indirectly accommodate all turning may be used in combination with a directional midblock movements. Figure 48 presents a conventional median open- median opening (Type 2a or 2b) to indirectly accommodate ing at a four-leg intersection (Type 5b) and a combination of a all turning movements. Figure 47 presents a conventional directional median opening at a four-leg intersection (Type 6a) median opening at a three-leg intersection (Type 3b) and a with two directional midblock median openings (Type 2b). combination of a directional median opening at a three-leg From Table 39, the median opening accident rate for a intersection (Type 4a) with a directional midblock median conventional median opening at a four-leg intersection (Type opening (Type 2b). 5a + 5b) is 3.01 accidents per million turning vehicles. From From Table 39, the median opening accident rate for a con- Tables 34 and 39, the median opening accident rate for the ventional median opening at a three-leg intersection (Type combination of directional median openings is 3.03 accidents 3a + 3b) is 2.69 accidents per million turning vehicles. From per million turning vehicles (2.57 for the Type 6a median Tables 35 and 39, the median opening accident rate for the opening plus 0.46 for two Type 2b median openings), which combination of directional median openings is 1.63 accidents is about equal to the median opening accident rate for the per million turning vehicles (1.40 for the Type 4a median conventional median opening. 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 Comparison of Conventional accident rate for a conventional median opening at a three- and Directional Median Openings at Signalized Four-Leg Intersections leg intersection. Although the focus of the research was on unsignalized Comparison of Conventional and Directional median openings, most of the directional midblock median Median Openings at Four-Leg Intersections openings in the study were sites in Michigan, where they are used in combination with signalized intersections. At these A conventional median opening at a four-leg intersection locations, left turns are prohibited from the major and minor (Type 5a, 5b, or 5c) accommodates all turning movements road at the signalized intersection. From the minor road, left- from the major road and cross street as well as through move- turning traffic turns right onto the divided road and then ments on the cross street. A directional median opening at a makes a U-turn at the directional midblock median opening. four-leg intersection accommodates only left turns from the Traffic turning left from the major road must first travel

OCR for page 74
77 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) Figure 47. Conventional and directional median openings at three-leg intersection. through the intersection and then make a U-turn at the direc- turning volumes, no formal comparison of the relative safety tional midblock median opening, followed by a right turn of the two scenarios (conventional and directional median onto the minor road. openings at signalized four-leg intersections) can be made. Figure 49 illustrates a conventional signalized four-leg However, given that the median opening accident rate for intersection and a combination of directional midblock median directional midblock median openings is so low, it is likely openings with a signalized intersection where turns are pro- that the safety performance of the directional median opening hibited. Without a formal safety prediction model for signal- combination, as used in Michigan, is as good as or better than ized intersections on divided highways that is sensitive to the safety performance of the conventional median opening. 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.

OCR for page 74
78 Traffic Signal a) Conventional Median Opening at Signalized Four-Leg Intersection Traffic Signal b) Signalized Four-Leg Intersection With No Turns Permitted and Two Directional Midblock Median Openings Figure 49. Median openings at signalized four-leg intersection.