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24 ing crash reductions for wider paved widths, wider lanes, and wider shoulders. More specific to the research objec- tive, CMFs were provided for various laneshoulder con- figurations. Individual state analyses did not indicate a clear preference for lane or shoulder width given a fixed paved width, but combined with findings from previous research, researchers described some potential trends: For 26- to 32-ft total paved widths, 12-ft lanes provided the optimal safety benefit. The CMF ranged from 0.94 to 0.97, indicating a 3% to 6% crash reduction for 12-ft lanes compared with 10-ft lanes. For 34-ft total paved width, 11-ft lanes provided the optimal safety benefit. The CMF for 11-ft lanes was FIGURE 4 Open priced dynamic shoulder lane 0.78 compared with the 10-ft baseline. (Credit: Minnesota Department of Transportation). For 36-ft total paved width, 11- or 12-ft lanes provided the optimal safety benefit. The CMF was 0.95 for 11- Lane Width and 12-ft lanes compared with the 10-ft baseline. Potts et al. (2007b) investigated the relationship between These results applied, in general, to rural, two-lane roads lane width and safety for roadway segments and intersection with traffic volumes greater than 1,000 vehicles per day and approaches on urban and suburban arterials. Their research posted speeds of 25 mph or greater. Although 12-ft lanes found no general indication that the use of lanes narrower than appeared to be the optimal design for 26- to 32-ft total paved 12 ft (3.6 m) on urban and suburban arterials increased crash widths, 11-ft lanes performed equally well or better than frequencies. Researchers stated that this finding suggested that 12-ft lanes for 34- to 36-ft total paved widths. geometric design policies can provide substantial flexibility for use of lane widths narrower than 12 ft (3.6 m). They added that The Highway Safety Manual (AASHTO 2010) provides inconsistent results suggested increased crash frequencies with CMFs for lane width on two-lane highway segments, which narrower lanes in three specific design situations: are presented in Table 8. The base value for the lane width CMF is 12 ft. For lane widths with 0.5-ft increments that Lane widths of 10 ft (3.0 m) or less on four-lane undivided are not depicted specifically in Table 8, a CMF value can be arterials. interpolated because there is a linear transition between the Lane widths of 9 ft (2.7 m) or less on four-lane divided various AADT effects. A corresponding chart is also pro- arterials. vided as a figure in the HCM. Lane widths of 10 ft (3.0 m) or less on approaches to four-leg stop-controlled arterial intersections. Number of Lanes The researchers recommended that "narrower lanes should be used cautiously in these three situations unless local experi- Kononov et al. (2008) explored the relationship between ence indicates otherwise." safety and congestion on urban freeways by examining the shape of the safety performance functions (SPFs). SPFs are Gross et al. (2009) studied a variety of crash data and crash prediction models that relate traffic exposure, measured roadway characteristics to determine the safety effectiveness in AADT, to safety, measured in the number of accidents over of specific combinations of lane and shoulder width on rural, a unit of time (e.g., accidents per mile per year). They found two-lane, undivided roads. In general, all else being equal, that to that point "the focus of most SPF modeling efforts results were consistent with previous research efforts, show- had been on the statistical technique and the underlying prob- Table 8 Crash Modification Factors for Lane Width on Roadway Segments AADT (vehicles per day) Lane Width 2,000 9 ft or less 1.05 1.05 + 2.81 10-4 (AADT 400) 1.50 10 ft 1.02 1.02 + 1.75 10-4 (AADT 400) 1.30 11 ft 1.01 1.01 + 2.50 10-5 (AADT 400) 1.05 12 ft or more 1.00 1.00 1.00 Source : AASHTO (2010) . Note : The collision types for which this CMF is applicable include single-vehicle run-off-road and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes.

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25 ability distribution, with only limited consideration given to with another Iowa study that used a simple before-and-after the nature of the phenomenon itself." Their relationship of approach on the same sites. safety to the degree of congestion suggested that safety deteri- orated with the degradation in the quality of service expressed NCHRP Report 617 (Harkey et al. 2008) presents the through the level of service. Their assessment was that prac- findings of a research project to develop CMFs for traffic titioners generally believed the additional capacity afforded engineering and Intelligent Transportation System improve- by additional lanes was associated with more safety, but how ments. One such improvement was the "road diet." Researchers much safety and for what time period were generally not con- estimated the change in total crashes owing to the conversion sidered. Comparison of SPFs of multilane freeways suggested and use Empirical Bayes methodology to compare the results that adding lanes may initially result in a temporary safety with previous studies. They reviewed geometric, traffic, and improvement that disappears as congestion increases. They crash data for 45 treatment sites and 347 reference sites in found that total as well as injury and fatal crash rates increased Iowa, Washington, and California, and found significant with AADT and that it was significantly safer to travel on effects on crashes. Their recommendations for CMFs are urban freeways that operate at level-of-service (LOS)-C or shown in Table 9. better during the peak period than on more congested facili- ties. As AADT increased, the slope of SPF, described by its first derivative, became steeper, reflecting that crashes were Resurfacing increasing at a faster rate than would be expected from a free- way with fewer lanes. As the number of lanes increased, so The research team on NCHRP Project 3-56 (Harwood et al. did the opportunity for drivers to maneuver around slower 2003a) developed a process for allocating resources to maxi- traffic. Increased maneuverability tended to increase the aver- mize the effectiveness of 3R projects in improving safety and age speed of traffic, but at the same time it increased the speed traffic operations on the nonfreeway highway network. They differential and the number of crashes related to lane changes, developed a program called the Resurfacing Safety Resource such as sideswipes and rear-end crashes. Allocation Program (RSRAP) designed to allow highway agencies to maximize the cost-effectiveness of the funds spent on 3R projects by improving safety on nonfreeway facilities Road Diet while maintaining the structural integrity and ride quality of the highway pavement. To do this, their process considered: Huang et al. (2002) investigated the effects on crashes and injuries through conversion of an undivided four-lane road "A specific set of highway sections that are in need of to three lanes and a TWLTL, also known as a "road diet." resurfacing either at the present time or within the rela- They reviewed before-and-after crash data from 12 road diet tively near future; sites and 25 comparison sites in California and Washington, A specific set of improvement alternatives for each can- and found that "the percent of road diet crashes occurring didate site, including doing nothing, resurfacing only, during the `after' period was about 6% lower than that of and various combinations of safety improvements for the matched comparison sites." However, a separate analy- the site; and sis in which a negative binomial model was used to control A limit on the funds available for improvements to the for possible differential changes in ADT, study period, and set of highway locations." other factors indicated no significant treatment effect. Crash severity was virtually the same at road diets and compari- The RSRAP procedure considers other treatments in addi- son sites, whereas there were some differences in crash type tion to resurfacing, such as lane width changes, turning lane distributions between road diets and comparison sites, they improvements, and shoulder widening. Among their find- found none between the "before" and "after" periods. They ings, the research team concluded that: concluded that conversion to a road diet should be made on a case-by-case basis in which traffic flow, vehicle capacity, Resource allocation methods provided an effective and safety are all considered. They also recommended that method for highway agencies to decide when safety the effects of road diets be further evaluated under a variety improvements are to be made in conjunction with pave- of traffic and roadway conditions. ment resurfacing projects. For a given set of sites, resource allocation methods Pawlovich et al. (2006) used a Bayesian approach to evalu- provided an optimal mix of resurfacing treatments with ate the effects of the "road diet" on crashes in Iowa. Their meth- and without accompanying safety improvements that odology incorporated both monthly crash data and estimated provided greater benefits than any fixed strategy. volumes for 30 sites--15 treatment and 15 comparison-- Resurfacing without accompanying geometric improve- for more than 23 years (1982 to 2004). Their results indicated ments may cause a small, short-term increase in acci- a 25.2% reduction in crash frequency per mile and an 18.8% dents resulting from increased speeds; however, the reduction in crash rate. The authors stated that their results evidence for this effect was conflicting. An optional from the Iowa study fit practitioner experience and agreed feature in the RSRAP software allowed the user to