Larger number of lanes

Most of the studies show that an increasing number of lanes is related to an increase in crashes. This might be partly explained by an increase in lane changing and overtaking manouevres and speed differences between vehicles. Another relationship is that a higher number of traffic lanes relates to a higher traffic demand. This means that the relation between number of lanes and crashes is not causal. The effect of the number of lanes on crashes always concerns the number of crashes or total crash reduction, often with a distinction between crash severities. A distinction between crash types is rarely found. One study indicates a decreasing number of crashes for an increase of lanes, while the remaining studies indicate the opposite. The difference is related to other factors like annual average daily traffic (AADT), speed limits, lane width, road type and the percentage of heavy good vehicles (HGV). Most of the studies involve Crash Prediction Models (CPMs).