Median Intersection Design for Rural High-Speed Divided Highways (2010) / Chapter Skim
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Pages 64-146
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From page 64... ...
64 Overview As described in the previous chapter, STAs have experimented with a wide range of intersection safety treatments at problematic rural expressway intersections. Unfortunately, literature on many of these treatments is scarce, so their safety effectiveness is relatively unknown and national geometric design guidance is lacking.
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maneuvers)
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From page 66... ...
rural expressway intersections is a more recent application, so national design guidance for the J-turn intersection is relatively non-existent. In fact, this type of intersection is only briefly mentioned in Chapter 9 of the AASHTO Green Book (3)
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From page 67... ...
J-turn intersections have already been constructed on rural expressways in Maryland, North Carolina, and Florida with other states like Iowa, Missouri, and Minnesota seriously considering their use on rural expressways. Some state design guidance on J-turns is available within the North Carolina DOT (NCDOT)
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68 Figure 50. MoDOT standard plan for J-turn intersection (30)
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able to suggest that the primary contributing factor to these crashes was related to gap recognition and selection by drivers on MD-313 attempting to cross or turn onto US-301. Over the years, the MSHA had installed additional signage at the intersection to address its historically poor safety record.
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STOP AHEAD warning sign shown in Figure 55C was placed overhead on both MD-313 approaches to alert drivers to the presence of the stop control and the divided highway ahead. However, these signs failed to improve the safety performance of the intersection.
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71 Figure 54. Collision diagram at US-301 and MD-313 before J-turn construction (67)
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the median if they wish. Therefore, all drivers on the MD-313 approaches are forced to turn right with indirect left-turn and crossing maneuvers accommodated via median U-turns located approximately 1,500 ft from the main intersection as shown in Figure 53.
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73 Figure 57. Collision diagram at US-301 and MD-313 after J-turn construction (67)
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installed during November 2000 (there were no crashes during these two months)
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are in this range and have instead been widening and thickening shoulders (constructing loons) to accommodate WB-67 turning paths.
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From page 76... ...
• Construction of a U-turn on the Exit 85 ramp approximately 1⁄2 mile to the west of the intersection; and • Posting of U-TURN TRAFFIC ENTERING warning signs on both US-23/74 approaches as well as other navigational guide signs; U-turns to the east are made at a previously existing intersection [US-23/74 and SR-1788 (Hidden Valley Road)
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From page 77... ...
before period was significantly reduced for total, injury, rightangle, far-side right-angle, and near-side right angle collisions as shown in Table 24. The second J-turn intersection safety evaluation conducted by the NCDOT was at the intersection of Business US-64 (Knightdale Boulevard)
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From page 78... ...
in the US-64 traffic stream. Of the 45,000 vehicles that used the intersection daily, approximately 400 went straight or turned left from the minor roads; however, these vehicles were involved in 12 of the 21 crashes (57%)
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79 Figure 61. Before and after collision diagrams at US-64 and Mark's Creek Road (70)
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south of Lenoir, North Carolina. US-321 is a four-lane divided highway with a posted speed limit of 55 mph, but this intersection is located in a more suburban environment than are the previous two examples, with more businesses located along US-321.
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From page 81... ...
• Slight modification of the raised right-turn channelization on both minor road approaches, • Extension of the left-turn storage lanes on the US-321 approaches, and • Posting of additional navigational guide signs. A quality aerial photo of this intersection was not available, but a location map is shown in Figure 62.
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From page 82... ...
82 Before and after collision diagrams are shown in Figure 63, while Table 26 summarizes the naïve before-after crash data comparison for the J-turn intersection conversion at US-321 and SR-1796. Overall, there was a 69% reduction in total crashes with reduced crash severity.
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From page 83... ...
end and left-turn collisions with opposing traffic were not statistically significant. Summary The assumed safety benefit of J-turn intersections is that they reduce the potential for right-angle collisions (particularly far-side right-angle collisions)
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From page 84... ...
left-turns leaving the mainline, crashes occurring within the median area are expected to be reduced. Finally, by physically separating and offsetting opposing left-turn lanes on the mainline, J-turn intersections may also help reduce collisions between opposing left-turn vehicles and other "left-turn leaving" crashes between left-turn vehicles leaving the expressway and opposing through traffic.
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From page 85... ...
by reducing, relocating, or controlling the number and/or type of vehicular conflicts that can occur at an intersection. The key to the effectiveness of these treatments, however, is in eliminating the high-risk conflict points.
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When two T-intersections combine to form an offset T-intersection configuration, the total number of conflict points is 26 (11 conflict points at each T-intersection plus 2 merge and 2 diverge points in between) , regardless of whether it is an R-L or an L-R configuration.
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From page 87... ...
adequate space to merge across the expressway lanes and safely enter the opposite minor roadway without causing undue interference to through expressway traffic. However, similar to the J-turn intersection, selection of the most appropriate separation/offset distance is a trade-off between providing sufficient space for safe and functional weaving areas and adequate left/right-turn storage while minimizing the travel distance and time of the indirect crossing maneuver from the minor road.
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From page 88... ...
T-intersection was created, but the original intent was not to create an offset T-intersection: it happened to occur during the corridor development process as a result of design convenience and a desire to reduce the skew of the two intersections. Because this was not a direct conversion of a four-legged expressway intersection into an offset T configuration, no before-after crash data exists to examine the effectiveness of a conversion.
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fidence (α = 0.10) , the mean annual crash frequency was significantly reduced in the after period for total and severe injury crashes as shown in Table 29.
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unequal variances and a 90% level of confidence (α = 0.10) , the reductioninfar-side right-angle crash frequency was statistically significant, but the changes in all other crash types were not.
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the importance of identifying opportunities to create offset T-intersections during the initial corridor planning process as rural two-lane undivided highways are being upgraded to divided highways. Finally, a typical signing plan for an offset T-intersection should be incorporated into the MUTCD.
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From page 92... ...
differentials in the passing lane may not be reduced as much as expected since left-turning vehicles exiting the expressway must first merge into the passing lane prior to entering the left-turn deceleration lane. One rural expressway intersection design alternative that removes left-turning vehicles from the high-speed expressway traffic stream while only allowing turns to be made from the right-hand lane is the "jughandle intersection." The New Jersey DOT (NJDOT)
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other)
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jughandle intersections are shown in Figure 74. An aerial photo of one such intersection is shown in Figure 75.
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quate storage, thereby preventing queue spillback onto the expressway.
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96 Figure 74. Example signage for jughandle intersections.
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used jughandles at expressway intersections. Unfortunately, none of the 11 STAs that indicated that they had used jughandle intersections on expressways was able to provide the requested before and after crash data.
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from the minor road, it is hypothesized that unsignalized jughandle intersections may increase the frequency of rightangle collisions. Unsignalized jughandle intersections (near-side and far-side)
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Previous research in Minnesota confirmed these findings. One study identified gap selection as the primary factor contributing to almost 60% of the crossing path crashes at rural intersections along two-lane highways in Minnesota (85)
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intersections, while the other three states (Georgia, Michigan, and New Hampshire) selected rural undivided highway intersections.
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were originally created. As a result, when a driver is stopped on the eastbound approach of CSAH-9, the elevated southbound lanes of US-52 block their view of northbound traffic (see Figure 80)
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date, driver gap selection behavior data has been collected at seven of these intersections. Overall, the vehicle surveillance system has been found to be highly accurate in detecting, tracking, and predicting the arrival times of mainline vehicles.
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IDS is expected to reduce right-angle collisions by utilizing real-time traffic conditions to inform minor road drivers when a safe gap in mainline traffic exists for making crossing or merging maneuvers. The system is still under development and may not be ready for deployment for a number of years, but less-sophisticated systems used in Virginia, Maine, and Georgia have been deployed at intersections on two-lane roadways and have improved safety (16, 35, 36)
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signing and marking technique at TWSC intersections on an extremely limited basis. Zwahlen et al.
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105 pictured in Figure 85, was at an unidentified TWSC intersection on a two-lane undivided rural highway. Notice the crosses on the road surface in front of the approaching vehicle, the roadside delineators, and the WAIT IF VEHICLE IN MARKED AREA sign.
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manual, informational brochures given out as drivers renew their license or purchase their vehicle registrations, etc.)
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107 showing how to use MALs is included within the appendix of a study conducted by Hanson (39)
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cautions that a MAL should not be excessively long because mainline through drivers may mistake it for an additional through lane and be compelled to enter into it. MnDOT has constructed MALs at rural expressway intersections, and Chapter 5 of the MnDOT Road Design Manual (29)
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the 48 responding U.S. transportation agencies (STAs and large municipalities)
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The MAL at US-54 and Business-54/Route W was constructed in Fall 1998 and the traffic signal was installed in January 2001, so, at this intersection, the before period is 4 years (1994–1997) and the after period is limited to 2 years (1999–2000)
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87.5% of all right-angle collisions (14 out of 16) after the MAL installation.
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The keys to successfully designing a MAL are providing adequate length and creating a median opening area that minimizes conflicts between vehicles entering the MAL and other vehicles using the median. MnDOT and MoDOT provide some design guidance in this regard, but national design guidance is needed and should be incorporated into the AASHTO Green Book.
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From page 113... ...
where right-turn volumes are substantial. However, the limited research assessing the safety effects of providing exclusive right-turn lanes at rural expressway intersections reveals that conventional right-turn lanes may actually increase crashes (2, 20)
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The most important design aspect of an offset right-turn lane is that it should provide the minor road driver with a clear departure sight triangle to the left (i.e., sufficient sight distance along the near-side expressway lanes) when rightturning vehicles are present on the mainline.
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AASHTO Green Book and need to be reexamined using the updated ISD criteria given in the current edition (3)
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Iowa Experience Two examples of offset right-turn lane installations on rural expressways were found in Iowa. The first example is located at the intersection of US-61 and Hershey Road near the western edge of Muscatine.
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From page 118... ...
Because two offset right-turn lanes were installed at this location, one on northbound US-61 and one on southbound US-61, a separate before-after comparison of near-side rightangle collisions was conducted for each offset right-turn lane. The results of this analysis are shown in Table 42.
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traffic volumes (2)
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• Current right-turn traffic volumes at the intersection met NCHRP Report 279 (93) volume warrants for a full-width right-turn lane, • Westbound right-turning traffic often used the paved shoulder to complete the turn, • A heavy volume of truck traffic was using 148th Street, and • Although ISD was adequate, the intersection is placed on a crest vertical curve such that westbound traffic on N-2 does not see the intersection until just over the crest as shown in the upper left of Figure 96.
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tance is 12 ft. In addition, the same project also constructed a divisional (splitter)
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the time of this collision, but if there was, the offset rightturn lane may have helped prevent a more severe crash. Summary The assumed safety benefit of offset right-turn lanes is that they eliminate the sight distance obstruction created by the presence of right-turning expressway vehicles positioned in a conventional right-turn lane, allowing minor road drivers to make better gap acceptance decisions when entering the near-side intersection.
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from intersection to intersection based on each intersection's unique geometry (skew, horizontal curvature, approach grades, design speed, stop bar placement, etc.) ; therefore, intersection design plans should be checked to ensure that adequate ISD is provided.
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by Khattak et al.
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From page 125... ...
treatments are used for the first time in a given area (16)
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126 Figure 100. Minimum left-turn lane offset calculation trigonometry diagram (27)
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sufficient sight distance along the opposing expressway lanes) while opposing left-turn vehicles are present.
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128 Figure 102. Offset left-turn lane designs in Illinois, Iowa, and Nebraska (7, 26, 31)
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consist of: 1) 120 feet of 15:1 taper to shift the turning traffic 8 feet from the through lane, 2)
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Case Studies of Implementation and Safety Effectiveness North Carolina Experience For the purpose of this case study, the NCDOT Safety Evaluation Group, a subdivision of its Traffic Safety Systems Management Section, conducted safety evaluations at two high-speed TWSC expressway intersections where offset leftturn lanes had been installed. These before-after spot safety evaluations were at the intersection of US-421 (Carolina Beach Road)
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From page 131... ...
Since the offset left-turn lane treatment is meant to reduce left-turn leaving and rear-end crashes on the mainline, these are considered to be the "target" crash types and are examined separately. In the before period, 12 of the 21 crashes (57%)
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were previously located and how a left-turn driver's sight-line may have been obstructed by opposing left-turn vehicles. The bottom portion of Figure 107 demonstrates the improved sight-line of a driver in the offset left-turn lane.
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presence of opposing left-turn vehicles in conventional left-turn lanes, thereby allowing left-turn drivers to make improved gap selection decisions when exiting the expressway. Expressway intersections most likely to benefit from offset left-turn lanes include • Intersections with a history of left-turn leaving collisions resulting from opposing left-turn vehicles on the mainline obstructing each other's sight lines, and • Intersections with large volumes of opposing left-turning traffic leaving the expressway.
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to the Green Book for clarification.
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median nose should be considered to provide an approaching left-turn driver with a better sense of where the intersection is ultimately located and to provide an opposing through driver with better visual delineation. Freeway-Style Advance Intersection Guide Signing Case Study Description To motorists, a rural expressway may appear to be a freeway.
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136 Type of Sign Minimum Size (mm) Minimum Size (inches)
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they have not conducted any studies regarding their safety effectiveness. One disadvantage of diagrammatic signage is that it may be difficult to fit street names or multiple shields (if a roadway carries multiple numbered routes)
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From page 138... ...
upgrades at eight intersections along US-52 between Rochester and Inver Grove Heights during late 2003 and early 2004. This project cost approximately $20,000, but the work was done by MnDOT maintenance forces, and this price tag only includes the cost of materials.
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From page 139... ...
tions, which is the crash type the countermeasure was meant to address. Right-angle collisions also decreased overall.
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From page 140... ...
Currently, the MUTCD instructs the user to use the same types of guide signs specified for use on conventional roads when posting guide signs at rural expressway intersections. Although safety effectiveness and volume warrants have yet to be determined, language should be added to the MUTCD that supports the use of freeway-style advance guide signs (with or without diagrammatic layouts as used in Nebraska)
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From page 141... ...
to implementation warrants, but an informal review of TWSC rural expressway intersections in Minnesota found that most problematic intersections had a minor road volume of at least 2,000 vpd or an expressway volume of at least 25,000 vpd. This indicates that, at either of these volume levels, the demand for gaps is beginning to exceed the number of safe gaps and that traffic engineers should consider implementing signing or other safety improvements at the intersection.
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advance of this intersection. Blacksmith Road is a two-lane undivided roadway that includes channelized right-turn lanes and STOP signs on both intersection approaches.
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angle variety and 29 were far-side right-angle collisions. In the after period, there were a total of 14 intersection-related collisions (1 fatal, 8 injury, and 5 PDO)
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State Route V and County Road 54-68, which are both two-lane undivided roadways that provide direct access to the small town of Linn Creek. Both of these roadways are stop-controlled at their intersections with US-54 while the medians are yield controlled.
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From page 145... ...
Summary The assumed safety benefit of providing dynamic advance intersection warning signs/flashers along the expressway approaches at TWSC rural expressway intersections is that they increase intersection conspicuity and alert the expressway driver to the actual presence of minor road traffic, thereby heightening the expressway driver's awareness and improving their reaction time should a minor road driver select an unsafe gap when entering the intersection. As a result, this strategy targets right-angle and rear-end collisions on the mainline.
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From page 146... ...
based on the positive results of the North Carolina experience, it is suggested that design guidance on this intersection safety strategy be included in the MUTCD and volume or crash experience guidelines be developed indicating when such a countermeasure should be considered. There is likely a lower volume threshold at which safety begins to deteriorate and the system should be installed, as well as an upper volume limit where the minor road detection loops are not necessary and the mainline flashers should be set to flash continuously.
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