Abstract
A large number of crashes occur on curves even though they account for only a small percentage of a system’s mileage. Excessive speed has been identified as a primary factor in both lane departure and curve-related crashes. A number of countermeasures have been proposed to reduce driver speeds on curves, which ideally result in successful curve negotiation and fewer crashes. Dynamic speed feedback sign (DSFS) systems are traffic control devices that have been used to reduce vehicle speeds successfully and, subsequently, crashes in applications such as traffic calming on urban roads. DSFS systems show promise, but they have not been fully evaluated for rural curves. To better understand the effectiveness of DSFS systems in reducing crashes on curves, a national field evaluation of DSFS systems on curves on rural two lane roadways was conducted. Two different DSFS systems were selected and placed at 22 sites in seven states. Control sites were also identified. A full Bayes modeling methodology was utilized to develop crash modification factors (CMFs) for several scenarios including total crashes for both directions, total crashes in the direction of the sign, total single-vehicle crashes, and single-vehicle crashes in the direction of the sign. Using quarterly crash frequency as the response variable, crash modification factors were developed and results showed that crashes were 5% to 7% lower after installation of the signs depending on the model.
Highlights
Dynamic speed feedback sign (DSFS) systems are one type of traffic control device that has been used to reduce vehicle speeds successfully and, subsequently, crashes in applications such as traffic calming on urban roads
The advantages of the full Bayes (FB) as compared to the empirical Bayes (EB) approach are noted as follows: Takes into account all uncertainties in the analysis Provides more detailed causal inferences
The study developed crash modification factors for dynamic speed feedback signs that were installed on rural two-lane curves at 22 sites across seven states
Summary
Horizontal curves are a particular safety concern since they have a disproportionate number of crashes compared to tangent sections. Glennon et al [1] reported that curves have approximately three times the crash rate of tangent section and the majority of those crashes are lane departures. Shankar et al [3] evaluated divided state highways without median barriers in Washington State and found a relationship between the number of horizontal curves per kilometer and median crossover crashes. Farmer and Lund [4] evaluated single-vehicle fatal and injury rollover crashes using logistic regression and found that the odds of having a rollover on a curved section were 1.42 to 2.15 times that of having a rollover on a straight section
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