Abstract
The identification of contributory factors to crash frequencies observed in different highway facilities can aid transportation and traffic management agencies to improve road traffic safety. In spite of the strategic importance of the national Portuguese road network, there are no recent studies concerned with either the identification of contributory factors to road crashes or Crash Prediction Models (CPMs) for this type of roadway. This study presents an initial contribution to this problem by focusing on the national roads NR-14, NR-101 and NR-206, which are located in Northern region of Portugal. They are two-lane single carriageway rural roads. This study analysed the crash frequencies, Average Annual Daily Traffic (AADT) and geometric characteristics of 88 two-lane road segments. The selected segments were 200-m-long and did not cross through urbanized areas. The fixed length of 200 meters corresponds to the road length used in Portugal to define a critical point. Data regarding the annual crash frequency and the AADT were available from 1999 to 2010. Due to the high number of zero-crash records in the initial database, the data were explored to identify the best statistical modelling approach to be adopted. The Generalized Estimating Equations (GEE) procedure was applied to 10 distinctive databases formed by grouping the original data in time and space. The results show that the different observations within each road segment present an exchangeable correlation structure type. This paper also analyses the impact of the sample size on the model’s capability of identifying the contributing factors to crash frequencies. The major contributing factors identified for the two-lane highways studied were the traffic volume (expressed in AADT), lane width, vertical sinuosity, and Density of Access Points (DAP). Acceptable CPM was identified for the highways considered, which estimated the total number of crashes for 400-m-long segments for a cumulative period of two years.
Highlights
The increasing number of traffic crashes in rural and urban roadways has created the need to develop strategies to help highway agencies reducing these events
The main objective of this study is to identify the factors that contribute to fatal and injury crash frequency for road segments on Portuguese national roads NR-14, NR-101 and NR-206, which are located in the Northern region of Portugal
The main results for the models generated in the cases of the 200-m-long and 400-m-long segments are presented in Tables 6 and 7, respectively
Summary
The increasing number of traffic crashes in rural and urban roadways has created the need to develop strategies to help highway agencies reducing these events. Registered and analyzed road crash data allows for the identification of the areas or sites where safety measures have a greater potential for success and effectiveness (Thomas et al 2003). The definition of effective safety measures, which are chosen to cope with the necessary reduction of road traffic crashes in a given roadway facility (segment or intersection), benefits from knowledge regarding estimates of the facility’s expected crash frequency along with the contribution of facility’s physical and operational characteristics to the expected safety performance. The common approaches that are used to provide current and future safety performance estimates of roadway segments or intersections are historical crash data, statistical models based on regression analysis, before-after studies, and expert judgments (Harwood et al 2000). O. da Costa et al Portuguese two-lane highways: modelling crash frequencies for different temporal
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