Carl Sundstrom

Curve Collisions: Road and Collision Characteristics and Countermeasures

Horizontal curves are relatively dangerous portions of roadway networks. Agencies optimizing the use of safety funds should be aware of characteristics of the collisions on those segments. However, few previously published articles attempted to characterize collisions on horizontal curves. This article describes an effort that characterized collisions reported to be on curves in North Carolina using the Highway Safety Information System. More than 51,000 North Carolina (NC) collisions on two-lane road curves were compared to collisions on all two-lane roads and on all roads. In doing so we gained a perspective of how well various curves perform relative to other road areas. We investigated numerous two-lane curve-collision types. Those most overrepresented included: collisions on grades, rural, severe injury or fatal, fixed object (particularly tree, ditch, and embankment), overturn, off-peak hours (particularly during darkness on unlighted roads), weekend, holiday periods, and wet, icy, or snowy pavement. The analysis also revealed that there were few short roadway segments (of 0.1- to 1-mile length) with more than 10 reported curve collisions in 3 years. This article provides recommendations from the literature to treat overrepresented collision types on horizontal curves. Based on the analysis findings, agencies should target countermeasures for the most common and overrepresented collision types.

Evaluation of GIS applications for horizontal curve data collection

Applications that use a geographic information system (GIS) are common and useful in the field of transportation. Horizontal curves are critical components on a roadway in their role as a transition between straight segments and in their potential as a safety hazard to motorists. Therefore, determining the characteristics of curves [both spatial (including location, length, radius) and nonspatial (including traffic volume, signage, pavement type)] is an important task. GIS applications have been developed for evaluating roadway alignments in general and some have focused on horizontal curve assessment. This paper reports on two benchmarking efforts. The first assessed and established the accuracy of three publicly available GIS applications for determining curve spatial characteristics. The second assessed the quality of available GIS roadway line work. The three GIS applications were evaluated for performance accuracy on the basis of a comparison with precisely drawn curves [with radii ranging from 30.5 ...

Crash modification factors for changes to left-turn phasing

This study estimated crash modification factors (CMFs) from before–after evaluations of two treatments targeted at reducing left-turn crashes at signalized intersections: (a) changes from permissive to protected–permissive phasing and (b) the implementation of a flashing yellow arrow for permissive left turns. Results of the first evaluation—based on 59 intersections in Toronto, Ontario, Canada, and 12 in North Carolina—indicated a substantial reduction in left-turn opposing through crashes, especially at intersections at which more than one leg was treated, and a small percentage increase in rear-end crashes. For the second evaluation (the implementation of the flashing yellow arrow)—based on data from 51 signalized intersections in Oregon, Washington State, and North Carolina—the results indicated a safety benefit at locations with some kind of permissive left-turn operation before and a disbenefit at locations that had a protected-only operation before. The study estimated the standard deviation of the distribution of the CMF in addition to the conventionally estimated standard error of the mean CMF value. For several CMFs, the standard deviation of the distribution was larger than the standard error of the mean value of the CMF and indicated a substantial variation in the effect of the treatment across different sites. This finding indicates the need for further research into the development of crash modification functions instead of CMFs and for the use of large treatment databases to undertake a more extensive disaggregate analysis of the safety effects. The finding also emphasizes the importance of providing a more explicit consideration of CMF variability in future editions of the Highway Safety Manual.

Evaluation of Safety Strategies at Signalized Intersections

This report presents crash modification factors (CMFs) for safety strategies at signalized intersections. CMFs are a tool for quickly estimating the impact of safety improvements. The report will be of particular interest to safety practitioners responsible for programming and implementing highway safety improvements at intersections.

Development of Crash Modification Factors for Uncontrolled Pedestrian Crossing Treatments

The objective of this study was to develop crash modification factors for four treatment types: rectangular rapid-flashing beacon (RRFB), pedestrian hybrid beacon (PHB), pedestrian refuge island (RI), and advance yield or stop markings and signs (AS). From 14 cities throughout the United States, 975 treatment and comparison sites were selected. Most of the treatment sites were selected at intersections on urban, multilane streets, because these locations present a high risk for pedestrian crashes and are where countermeasures typically are needed most. For each treatment site, relevant data were collected on the treatment characteristics, traffic, geometric, and roadway variables, and the pedestrian crashes and other crash types that occurred at each site. Cross-sectional regression models and before–after empirical Bayesian analysis techniques were used to determine the crash effects of each treatment type. All four of the treatment types were found to be associated with reductions in pedestrian crash risk, compared with the reductions at untreated sites. PHBs were associated with the greatest reduction of pedestrian crash risk (55% reduction), followed by RRFBs (47% reduction), RIs (32% reduction), and AS (25% reduction). The results for RRFBs had their basis in a limited sample and must be used with caution.