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Impact of Access Spacing Standards on Crash Risk After Controlling for Access Volumes
Alejandra Medina Flintsch, Feng Guo, Shu Han, Kathleen Hancock, Brian Williams, Yingfeng Li, and Ron Gibbons
Year: 2019
VTRC No.: 21-R7

Engineers and planners are typically tasked with approving new entrance permits, evaluating permitting exceptions, and generating safety countermeasures for specific corridors. Although several studies show that a smaller space between two access points is associated with an increased crash risk, the relationship between these two factors is not fully understood. The Virginia Department of Transportation is interested in determining the impact of access spacing on crash risk as a result of controlling by access volume. This study encompasses an effort to understand how the spacing between access points and volume of access points affect crash risk. The three tasks were to conduct a literature review to identify research gaps, a pilot study to evaluate the feasibility of the approach, and an extended study to use the pilot study results to provide recommendations for application in the field.

The pilot study helped to develop the methodologies for data collection, information extraction, and statistical analysis. It also raised discussion on the definition of analysis units. As a result, three alternative definitions were used in the extended study. Eight corridors totaling 621 miles were selected for the extended study. Several existing databases and tools were used to determine roadway geometric attributes, operational attributes, and traffic volume data for the selected corridors and access points. A new Linear Referencing System was generated to enable correlations between the different databases. A custom in-house application was developed in which a data reductionist used satellite imagery to determine the physical characteristics of an access point and the types of business and residential buildings connected to an access point. The application then used the estimated daily traffic volume from the ITE Trip Generation Manual in order to estimate traffic volume for the access point. The result of the reduction effort was a new database which included a list of access point pairs, physical variables, and spacing. Using this data along with the new Linear Referencing System, Poisson and negative binomial (NB) mixed regression models were used to evaluate the effects of access spacing and access traffic volume on crash risk.

The study concluded that both spacing and access volume have a significant impact on crash risk at/near access points. Access volume, including both the volume from upstream and downstream access points,has a positive association with crash risk. On average, for every 457 vehicles per day increase, the crash rate will increase by 4%–10% depending on the type of access points and analysis unit. Consistent with the majority of previous studies, access spacing was found to be negatively associated with crash risk. For every 100-foot increase in spacing, the crash rate will decrease by 4%–7%. Both access volume and spacing should be considered in developing access point standards and decision making in access management practice.