Evaluating the Road Safety Effects of Interchange Spacing: A Multidisciplinary Approach

In the course of a Preliminary Design Study for the upgrade of an existing two-lane, two-way highway on the edge of a local mid-size community in Southern Ontario, the ultimate plan evolved into a fully controlled-access highway. Under the proposed scheme, existing intersections with arterial roadways were to be eliminated and all access to and from the facility was to be provided through interchanges. Roads crossing the facility that did not provide access were to be either grade-separated or terminated on either side of the highway. At one such location, the local community asked that consideration be given to providing a full interchange instead of the proposed flyover. In reviewing this request, concerns were raised with respect to the safety and operations of the proposed controlled access facility if an interchange were to be inserted in the requested location because of the close proximity of the adjacent upstream and downstream interchanges. This paper provides an overview of an innovative and practical approach to the explicit evaluation of the likely road safety performance impact of the requested interchange. We begin with a brief overview of the overall controlled access facility and the design context of the requested interchange. The analysis framework, which requires the development of a risk commentary along each of 5 independent lines of evidence (Interchange systems design practices; human factors; surrogate safety measures assessment through simulation; road safety engineering considerations; and quantitative operational analysis), is then presented. In our final analysis, we connect the various lines of evidence through an integrated and qualitative discussion of the commonalities and differences of the various indicators examined. Where lines of evidence “overlap” and point to a common conclusion regarding a particular feature or element of the interchange, that conclusion is strengthened by the independence of the indicators and the multiplicity of their occurrence as well as the independence of the individual investigators pursuing the different approaches to the analysis. The approach results in a set of clear and practical recommendations that are easily understood and appreciated by both technical and non-technical audiences alike.