Safety Evaluation of Dual Left-Turn Lane Installations in North Carolina

Crash modification factors (CMFs) were calculated for the conversion of a single left-turn lane to a dual or double left-turn lane (DLTL). Despite their proliferation throughout urban and suburban traffic networks, the safety performance of intersection approaches with DLTLs has not been thoroughly investigated and documented within the library of transportation safety research. To date, no published research has been completed that recommends a CMF for DLTL installations. This research effort lays a foundation for the understanding of the before-after safety effect of this countermeasure. A sample of 36 signalized intersections in North Carolina were investigated that received DLTLs between 2004 and 2021. Of these intersections 18 received their DLTLs with no other accompanying geometric changes and were operating with protected left-turn phasing along the treated approaches before and after their DLTL installations. A before-after evaluation of these 18 sites using an empirical bayes methodology yielded the following CMFs: 0.974 (total crashes), 0.844 (fatal-and-injury crashes), 1.010 (property damage only crashes), 0.831 (frontal impact crashes), 0.951 (rear end crashes), and 1.241 (sideswipe crashes). This research recommends that the CMFs for fatal-and-injury crashes and property damage only crashes be used in future cost–benefit calculations when planning DLTL installations.