Design Guidance for Intersection Auxiliary Lanes (2014) / Chapter Skim
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Pages 77-100
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From page 77... ...
77 Deceleration Field Study Background The objective of NCHRP Project 03-102 was to recommend improvements to the guidance provided in the AASHTO Policy on Geometric Design of Highways and Streets (commonly known as the Green Book)
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78 These variables were used to identify the effects of taper and deceleration length and posted speed limit. Literature AASHTO Policies on Deceleration into a Left-Turn Lane The 2011 Green Book (2)
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79 had slightly different deceleration lengths; however, the references were also to the 1998 course notes (84) referenced in the 2011 Green Book.
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80 using an initial speed of 55 mph. Test conditions included enabled and disabled antilock brakes, wet or dry pavement conditions, and two geometric conditions (tangent section and horizontal curve)
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81 presented in Figure 6-2a and distributions for the deceleration rates of the two approach speed categories are plotted in Figure 6-2b. Figure a shows a strong upward trend between deceleration rate and approach speed.
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82 authors recommended that the no-speed-reduction lengths given in Table 6-4 should be accepted as a desirable goal and should be provided where practical. Warren et al.
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83 In addition to the main study factors (i.e., posted speed limit and taper length) , the field study also controlled other important variables that might affect the operation such as those in Table 6-6 so as to limit their effects on the MOEs and to allow the effects of posted speed limit and taper length to be more easily identified.
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From page 84... ...
84 potential sites in Alabama, Florida, Mississippi, North Carolina, Tennessee, and Texas was conducted to produce a list of viable candidate sites. More than 70 sites in these six states were considered during site selection, and a list of candidate sites was developed for researchers to visit in person and collect data at the most promising sites.
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85 vided a permanent recording of the conditions and events at the site, speed counters provided the vehicle speeds upstream of the site, and the GPS and lidar provided a way to obtain comparative speed data to use for quality control. Traditionally, video is not as commonly used to collect this type of speed data as the other methods, but the research team conducted preliminary field tests and compared the relative benefits of each method in selecting the video method for this study.
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86 adjacent landowners. The length of the study period at each site was generally between 3 and 4 hr.
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87 the length of the left-turn lane or the available roadside area to position the trailer mandated that the cameras be aimed to exclude a middle portion of the left-turn lane, typically between points C and F Using this method, researchers were able to maintain sufficient visibility on the extents of the leftturn lane and key points while omitting a short length from the field of view that was not necessary for data reduction.
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88 reviewer then opened the video file and, using field notes recorded at the study site, identified the location of each key point on the video screen. Field technicians at each site walked across the roadway at each key point, which was recorded in the video.
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89 vehicle would be visible in the video. To identify a particular left-turning vehicle documented in the video, researchers established the presence of one or more nearby vehicles; focusing on identifying non-passenger vehicles (e.g., buses and semi-trailers)
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90 Speed Differential of Left-Turning Vehicles The first research question in this study focused on leftturning vehicles' difference in speed between the upstream traffic counter and the end of the taper. To determine whether the differential from Note 3 in Table 6-10 is representative of current drivers, researchers analyzed the differences in speed between the upstream traffic counter and the left-turn taper for the 410 left-turning vehicles observed in the video that had corresponding speed readings from the traffic counter.
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91 Table 6-12. Speed differential by site.
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92 and notes in Table 9-22, the Green Book (2) adds the following text on page 9-127: Inclusion of the taper length as part of the deceleration distance for an auxiliary lane assumes that an approaching turning vehicle can decelerate comfortably up to 10 mph before clearing a through lane.
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93 Referring to the length of the auxiliary lane, a relationship between shorter lengths and higher speed differentials, as stated in the previous Green Book quotation, is intuitive. Indeed, the two largest speed differentials are associated with sites that have the greatest differences in length compared to Green Book recommendations; however, a strong statistical relationship did not appear in this dataset, as shown in Table 6-15 and Figure 6-9.
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94 significant at a = 0.05 (see Table 6-15)
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95 tion on the models that considered speed limits in separate categories. Those models are shown in Table 6-17 and 6-18.
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96 magnitudes. This is a function of the units in which the variables are reported; a 100-ft increase in deceleration length is associated with a decrease in deceleration rate of about 1.7 ft/s2, while a 1-mph increase in upstream speed is associated with an increase in deceleration rate of about 0.8 ft/s2.
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97 lengths. The 5.8 ft/s2 value in Table 9-22 is used in calculating what is called the "full deceleration length" in Figure 9-48 (Figure 6-1 in this document)
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From page 98... ...
98 many locations, "it is not practical to provide the full length of the auxiliary lane for deceleration due to constraints such as restricted right-of-way, distance available between adjacent intersections, and extreme storage needs," which is why there is an allowance for deceleration in the through lane upstream of the taper.
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99 results of that analysis, in which deceleration length and the vehicle speed in the taper were found to be significant. Deceleration length is negatively related to deceleration rate, while speed is positively related; both of those relationships are as expected.
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100 Source: Access Management Manual (draft second edition) , exhibit 14-2b.
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