Access Management in the Vicinity of Interchanges, Volume 2: Research Overview (2022) / Chapter Skim
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Pages 84-100
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From page 84... ...
84 The evaluation of 26 interchange locations (52 terminal intersection configurations) with a wide variety of operational and geometric characteristics supports the approach of using safety case studies for this evaluation.
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Safety Analysis 85 tables that depict the percent of crashes based on collision type and crash severity. The tables also include some of the associated roadway information as well as the average number of crashes per year (to aid in understanding the scale of safety issues for each site)
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86 Access Management in the Vicinity of Interchanges Crash and Roadway Characteristics Total Crashes (%) AR #1 AR #2 AZ #2 AZ #3 KS #1 KS #2 MN #1 MN #2 MN #3 TX #1 Crash Severity Fatal (K)
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Safety Analysis 87 Crash and Roadway Characteristics Total Crashes (%) AR #4 VA #1 Crash Severity Fatal (K)
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88 Access Management in the Vicinity of Interchanges head-on collisions. This is particularly notable since this type of crash is a key concern when constructing a DDI.
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Safety Analysis 89 Figure 21. Example overview schematic for Site AR #3.
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90 Access Management in the Vicinity of Interchanges Conflict Studies In addition to the crash type and severity, it is possible to gain valuable insights into the influence of access point placement based on commonly observed vehicle conflicts. These conflicts can be expected to differ when the corridor permits unrestricted left-turn activity (such as an undivided road or a road with a TWLTL median)
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Safety Analysis 91 Figure 24. Example overview schematic documentation of crashes at driveways (AR #2)
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92 Access Management in the Vicinity of Interchanges Figure 25. More complex section of the 2011 AR #2 collision diagram.
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Safety Analysis 93 Figure 27. Conflicts between bicycle and driveway traffic.
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94 Access Management in the Vicinity of Interchanges First Access Point Downstream of the Interchange Terminal The placement of a DSDW in close proximity to an interchange terminal can introduce a variety of potential conflicts when the driveway location does not provide enough distance to accommodate essential vehicle acceleration distances or space to accommodate potential vehicle weaving maneuvers. Potential conflicts associated with the initial driveway located downstream of the interchange terminal can include • Conflicts between through vehicles turning right into the driveway and vehicles exiting the terminal intersection exit ramp and turning right to merge into through traffic, • Terminal intersection exit ramp right-turning vehicles merging into the right through lane and then immediately turning right into a driveway (introducing conflicts between the through traffic and slower-turning vehicle)
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Safety Analysis 95 Figure 30 depicts some of the vehicle conflicts observed in the vicinity of the first DSDW. The project team extracted the photo shown in Figure 31 from video data at AR #4.
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96 Access Management in the Vicinity of Interchanges First Access Point Upstream of the Interchange Terminal For a crossroad that does not have a median or has a TWLTL median, the placement of the first driveway upstream of the interchange terminal intersection should include a long enough spacing distance to simplify driver tasks and reduce potential conflict points. Based on field observations, the project team observed the following potential conflicts at an initial USDW location: • Conflicts can occur between driveway traffic and through traffic on the crossroad.
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Safety Analysis 97 the first driveway located upstream of the intersection. If the spacing to the first driveway is too short, a queue forms and ultimately spills into the adjacent through lane or the terminal intersection.
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98 Access Management in the Vicinity of Interchanges Figure 35. Crossroad queue spillback blocks driveway traffic (TX #2)
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Safety Analysis 99 Closely Spaced, Opposite-Side Upstream and Downstream Access Points (on Same Leg) The placement of the first USDW and the first DSDW on the same approach should be considered collectively.
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100 Access Management in the Vicinity of Interchanges Figure 40. Two-direction, overlapping left-turn movements on TWLTL (AR #4)
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