Per Gårder

Safety Effect of Roundabout Conversions in the United States: Empirical Bayes Observational Before-After Study

Modern roundabouts are designed to control traffic flow at intersections without the use of stop signs or traffic signals. U.S. experience with modern roundabouts is rather limited to date, but in recent years there has been growing interest in their potential benefits and a relatively large increase in roundabout construction. This interest has created a need for data regarding the safety effect of roundabouts. Changes in motor vehicle crashes following conversion of 23 intersections from stop sign and traffic signal control to modern roundabouts are evaluated. The settings, located in seven states, are a mix of urban, suburban, and rural environments with the urban sample consisting of both single-lane and multilane designs and the rural sample consisting of only single-lane designs. A before-after study was conducted using the empirical Bayes procedure, which accounts for regression to the mean and traffic volume changes that usually accompany conversion of intersections to roundabouts. For the 23 intersections combined, this procedure estimated highly significant reductions of 40 percent for all crash severities combined and 80 percent for all injury crashes. Reductions in the numbers of fatal and incapacitating injury crashes were estimated to be about 90 percent. In general, the results are consistent with numerous international studies and suggest that roundabout installation should be strongly promoted as an effective safety treatment for intersections. Because the empirical Bayes approach is relatively new in safety analysis, the potential of this methodology in the evaluation of safety measures is demonstrated.

THE IMPACT OF SPEED AND OTHER VARIABLES ON PEDESTRIAN SAFETY IN MAINE

The aim of this paper is to analyze pedestrian crashes with an emphasis on how actual travel speeds and characteristics of the locations influence crash numbers. Statewide data from Maine was analyzed. Descriptive crash and behavioral statistics are presented in the paper. Pedestrian and vehicle volumes were gathered for 122 locations in varying environments throughout Maine and crash numbers were predicted and compared to outcomes. Prediction models from Sweden and the UK were used since US models are nonexistant. It was found that high speeds and wide roads lead to more crashes and that the focus of safety improvement should be on arterials and major collectors. A strong relationship was also found between crash severity and speed.

MEASURING THE SAFETY EFFECT OF RAISED BICYCLE CROSSINGS USING A NEW RESEARCH METHODOLOGY

Before-and-after study methodology was developed and applied to evaluating the effect on bicyclists’ safety of raising urban bicycle crossings by 4 to 12 cm. In total, 44 junctions were reconstructed in this way in Gothenburg, Sweden. Four of these were studied in detail. Before the implementations, bicyclists were riding either in the roadway or on separate paths parallel to the roadway. The paths then ended with short ramps or curb cuts at each cross street, and bicyclists used nonelevated, marked bicycle crossings, similar to pedestrian crosswalks but delineated by white painted rectangles rather than zebra stripes. The results show that the paths with raised crossings attracted more than 50 percent more bicyclists and that the safety per bicyclist was improved by approximately 20 percent due to the increase in bicycle flow, and with an additional 10 to 50 percent due to the improved layout. However, the increased bicyclist volume means that the total number of bicycle accidents is expected to increase. Besides accident analysis, the change in risk was estimated using four different methods: surveys of bicyclists and experts, respectively; conflict data; and a quantitative expert model. Using a Bayesian approach for combining the results shows that the most likely effect of raising the bicycle crossing is a risk reduction of around 30 percent, compared with the before situation with a conventional bicycle crossing. Motorists and pedestrians also saw safety benefits from this traffic-calming measure.

AN EXPERT JUDGMENT MODEL APPLIED TO ESTIMATING THE SAFETY EFFECT OF A BICYCLE FACILITY

This paper presents a risk index model that can be used for assessing the safety effect of countermeasures. The model estimates risk in a multiplicative way, which makes it possible to analyze the impact of different factors separately. Expert judgments are incorporated through a Bayesian error model. The variance of the risk estimate is determined by Monte-Carlo simulation. The model was applied to assess the safety effect of a new design of a bicycle crossing. The intent was to gain safety by raising the crossings to reduce vehicle speeds and by making the crossings more visible by painting them in a bright color. Before the implementations, bicyclists were riding on bicycle crossings of conventional Swedish type, i.e. similar to crosswalks but delineated by white squares rather than solid lines or zebra markings. Automobile speeds were reduced as anticipated. However, it seems as if the positive effect of this was more or less canceled out by increased bicycle speeds. The safety per bicyclist was still improved by approximately 20%. This improvement was primarily caused by an increase in bicycle flow, since the data show that more bicyclists at a given location seem to benefit their safety. The increase in bicycle flow was probably caused by the new layout of the crossings since bicyclists perceived them as safer and causing less delay. Some future development work is suggested. Pros and cons with the used methodology are discussed. The most crucial parameter to be added is probably a model describing the interaction between motorists and bicyclists, for example, how risk is influenced by the lateral position of the bicyclist in relation to the motorist. It is concluded that the interaction seems to be optimal when both groups share the roadway.

Analysis of Factors Affecting the Severity of Head-on Crashes Two-Lane Rural Highways in Connecticut

The National Center for Statistics and Analysis and NHTSA suggest that head-on crashes are disproportionately represented in fatal crashes on two-lane highways, which constitute a substantial proportion of the highway network in the United States. This study focuses on analysis of the statistical association between head-on crash severity and potential causal factors, such as the geometric characteristics of the road segment, weather conditions, road surface conditions, and time of occurrence. Ordered probit models were estimated for data sets describing two-lane roads in Connecticut between 1996 and 2001. It was found that a wet roadway surface and narrow road segments were significantly related to more severe head-on crashes. A high density of access points and a nighttime occurrence of the crash were significantly related to more severe cases. Pavement width was found to be the most consistent factor, possibly because a wider road offers more space to avoid a direct head-on impact, thus reducing the severity of the crash. In addition, vehicle braking performance was important, as suggested by the higher probability of more severe head-on crashes on wet surfaces. The analysis results may be used by practitioners to understand the trade-off between geometric design decisions and head-on crash severity. Furthermore, identification of related factors will help to better explain crash phenomena and in turn to institute safer roadway design standards.

Safety Effect of Continuous Shoulder Rumble Strips on Rural Interstates in Maine

In Maine, the first rumble strips were installed in 1994, and the Maine Department of Transportation and the Maine Turnpike Authority have now installed continuous shoulder rumble strips (CSRS) on the majority of the rural sections of the Interstate and turnpike system. There were approximately 620 freeway miles with CSRS at the end of 2004. The purpose of this study is to report on the effectiveness of these installations in preventing run-off-road (ROR) crashes. An evaluation of their effectiveness showed that CSRS reduced sleep-related ROR crashes by approximately 58%. A statistical analysis showed that there is 99.9% certainty that the typical reduction in sleep-related ROR crashes was at least 41%. Fatal crashes were reduced to a higher degree than other crashes. Dry road ROR crashes were reduced by about 43% making the cost-benefit ratio of installing CSRS on a typical Maine section at least 195. CSRS were less effective in eliminating crashes during inclement weather conditions. The overall effecti...

Explaining pedestrian safety experience at urban and suburban street crossings considering observed conflicts and pedestrian counts

This article presents an investigation of pedestrian conflicts and crash count models to learn which exposure measures and roadway or roadside characteristics significantly influence pedestrian safety at road crossings. Negative binomial and ordered proportional odds technique were used to estimate pedestrian conflict count and severity models. Pedestrian counts and conflicts data were collected at 100 locations throughout Connecticut. Pedestrian crash data were collected from the Connecticut Crash Data Repository. The results show that minor and serious conflicts are marginally significant in predicting total pedestrian crashes together with crossing distance and building setback. This suggests that these conflicts may be a good surrogate for crashes in analyzing pedestrian safety. Greater crossing distance and small building setbacks are found to be associated with larger numbers of pedestrian–vehicle crashes. This latter effect is not expected, because we expect vehicle speeds to be lower in areas where the building setback is small. This factor may account for the greater pedestrian activity and more complex interactions in such areas. Further research aimed at identifying a minimum length of time for accurate estimation of pedestrian volume and conflicts to relate to crashes is the subject of continuing investigation by the authors.

Maine Statewide Deployment and Integration of Advanced Traveler Information Systems

Advanced traveler information systems (ATIS) are new to the state of Maine, and there are many unanswered questions about their effect on motorists. Dynamic message signs, variable speed limit signs (VSLS), and overheight vehicle detection (OHVD) systems are key components of ATIS and are the means through which motorists can be provided with en route information pertinent to their travels. Speed data were collected during inclement weather events to determine the effect of VSLS on traveler speed. Speeds were collected during storms of different intensities. The data suggest that motorists adjust their speed to conditions and what they think is appropriate. This finding is also supported by survey responses. The fact that motorists do not slow down to the posted speed could be partly because of the VSLS not being enforced. An OHVD system installed in Bangor, Maine, in August 2006 has shown potential, with no hits to the I-95 overpass since installation. Part of the study consisted of identifying and evaluating the institutional issues and barriers associated with intelligent transportation system deployment. These issues include long-term funding commitments for ATIS, acceptable messaging, integration of information databases, interagency coordination, enforcement, and education of the public.

Little Falls, Gorham: Reconstruction to a Modern Roundabout

Older American traffic circles can typically handle high traffic volumes, but they often have poor safety records. Modern roundabouts are similar to traffic circles in the sense that traffic “circles” around a central island, but many other design concepts differ. The first modern roundabout in Maine was opened to traffic in July 1997. The total accident rate after 16 months of operation is below that which would be expected if the junction had been signalized. There still has not been a single accident with injuries, and the injury rate is expected to stay significantly below that of a signalized intersection. The one-lane design will be able to handle traffic growth for years to come. Vehicle exhaust has been reduced, and delay on the minor approaches has been reduced by approximately 80 percent during morning and afternoon peak hours. In total, the reconstruction will save 5,000 to 10,000 h of travel time annually. This alone will pay for the investment within 5 years.

Left-Turn Gap Acceptance Behavior of Elderly Drivers at Unsignalized Intersections

This article describes an investigation of possible differences in elderly drivers’ gap acceptance behavior from other drivers when making left turns across oncoming traffic at unsignalized intersections. Elderly and other drivers were observed turning left at two unsignalized intersections with different speed limits of major roads. Statistical analyses were used to identify whether older drivers choose different gaps for left turns. Results indicated that drivers older than age 70 were different from drivers younger than age 35 and drivers age 55 to 69 in gap selection and female drivers different from male drivers. Finally, traffic simulations were run in VISSIM to determine how age differences in gap acceptance impact traffic operations. Results showed significant differences in the delay time, the number of stops per vehicle, and the total delay time of the network for drivers older than age 70 and other drivers under the same traffic conditions.