The results of research involving a well-designed before-and-after evaluation of the safety effects of providing left- and right-turn lanes for at-grade intersections are presented. Geometric design, traffic control, traffic volume, and traffic accident data were gathered for a total of 280 improved intersections as well as 300 similar intersections that were not improved during the study period. The types of improvement projects evaluated included installation of added left-turn lanes, added right-turn lanes, and extension of the length of existing left- or right-turn lanes. An observational before-and-after evaluation of these projects was performed by using several alternative evaluation approaches. Three contrasting approaches to before-and-after evaluation were used: the yoked comparison or matched-pair approach, the comparison group approach, and the empirical Bayes approach. The research not only evaluated the safety effectiveness of left- and right-turn lane improvements but also compared the performances of these three alternative approaches in making such evaluations. The research developed quantitative safety effectiveness measures for installation design improvements involving added left-turn lanes and added right-turn lanes. The research concluded that the empirical Bayes method provides the most accurate and reliable results. Further use of this method is recommended.
The relationship of safety to five candidate measures of geometric design consistency for rural two-lane highway alignments was examined. The measures that were evaluated included: ( a) speed reduction on a horizontal curve relative to the preceding tangent or curve, ( b) average radius, ( c) ratio of maximum radius to minimum radius, ( d) average rate of vertical curvature, and ( e) ratio of individual curve radius to average radius. All of these measures were found to have statistically significant relationships to accident frequency in the direction expected and are recommended as candidate measures for assessment of geometric design consistency. The relationships between design consistency measures and accident frequency were developed using Poisson, negative binomial, and lognormal regression analysis.
The safety effects of horizontal curves and grades on highways have been quantified separately, but it is not currently known whether or how the safety performance of horizontal curves interacts with that of grades. Although the first edition of the AASHTO Highway Safety Manual provides crash modification factors for the safety effects of horizontal curvature and percent grade on rural two-lane highways, the manual does not have a method for accounting for the interactions between these effects. In other words, in the Highway Safety Manual procedures for rural two-lane highways, the safety effect of a horizontal curve is the same whether the curve is located on a level roadway, a straight grade, or a vertical curve. Similarly, the safety effect of a straight grade is the same whether it is located on a tangent roadway or horizontal curve. Researchers have always supposed that there are interactions between the safety effects of horizontal and vertical alignments, but these interactions have not been demonstrated in a form useful for safety prediction. The results of research undertaken to quantify the safety effects of five types of horizontal and vertical alignment combinations based on Highway Safety Information System data and crash records from 2003 to 2008 in Washington State are summarized. The outcome is a set of safety pre diction models for fatal and injury and property-damage-only crashes. To present the results in a form suitable for incorporation into the Highway Safety Manual, crash modification factors representing safety performance relative to level tangents were developed from these models for each of the five combinations.
The service life of durable pavement markings was evaluated over a period of nearly 4 years. The evaluation included 85 study sites in 19 states at which a total of 362 longitudinal pavement marking lines (edgelines, centerlines, and lane lines) were installed. Marking materials included epoxy, flat and profiled polyester, flat and profiled poly(methyl methacrylate), flat and profiled thermoplastic, and profiled preformed tape; glass beads; and standard and snow-plowable raised retroreflective pavement markers. One site with conventional paint markings and three sites with waterborne paint markings were included for comparison. The service life of marking materials and roadway types was modeled as a function of time and cumulative traffic passages. Regression modeling identified large variations in the shape of the relationship for identical materials and types of lines between different sites within a given state and required that modeling be done separately for each pavement marking line at each study site. Variations in service life can be attributed to roadway type, region of the country where the markings were installed (e.g., weather conditions, manufacturers of marking materials and glass beads, and individual state highway agencies), marking specifications, contractors installing the markings, quality control at the time of installation, and winter maintenance snow removal policies.
Congestion on urban freeways often creates a need to increase freeway capacity by adding an additional lane. Although adding a lane by widening the existing roadbed is often difficult and expensive, restriping the traveled way with narrower lanes, converting all or part of the shoulder to a travel lane, or a combination of both often is a practical solution. An observational before-and-after evaluation with the empirical Bayes method was done to examine the safety effects of projects involving narrower lanes or shoulder conversions on existing urban freeways in California with four or five lanes in one direction of travel. The evaluation found that projects converting four lanes to five lanes resulted in increases of 10% to 11% in accident frequency. Projects converting five lanes to six lanes resulted in smaller increases in accident frequency. These increases in accident frequency may be the result of accident migration caused by relocation of traffic operational bottlenecks.
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