The impact of traffic flow distribution over arms at junctions on crash risk

In locations where primary and secondary roads cross, the distribution of traffic flow over the arms of a junction can introduce a potential road safety risk. Although the road traffic flow is a frequently considered variable, it is not easy to make a definitive conclusion about the specific effect that the distribution of traffic flow over the arms of a junction has on safety outcomes. This is due to the different variables that published studies use to express the specific risk factor. The aim of the present research is to (i) provide a literature overview of the phenomenon and to (ii) attempt to overcome said uncertainty by conducting a meta-analysis on the effects of traffic flow distribution over arms at junctions. Findings show that where there is an increase in: (i) the traffic volume on the minor or major road, or (ii) the number of turn lanes, crash frequency tends to increase. Where there is a significant flow imbalance between the junction branches (i.e. major and minor roads), mixed results were found in the literature, with crash rates both increasing and decreasing. Meta-analysis findings show that the amount of traffic flow of the secondary road can result in an increase in the number of crashes at a 95% confidence level. This conclusion can be exploited to inform principles of junction design that can consequently improve road safety.