Optimizing speed change lane lengths for two-way ten-lane expressways: a VISSIM and SSAM simulation-based approach

With rapid economic development, expressway traffic volumes have surged, resulting in severe congestion. To address this, expanding expressways to two-way ten-lane configurations has become a primary strategy for alleviating traffic pressure. Given the absence of unified national design standards for such highways, this study employs VISSIM microscopic traffic simulation integrated with the Surrogate Safety Assessment Model (SSAM) to investigate the relationship between speed change lane geometries and their design parameters. We evaluate the impacts of varying speed change lane lengths on traffic efficiency and safety under multiple traffic flow conditions for renovated two-way ten-lane expressways. Performance metrics include average delay time, average queue length, conflict frequency, and Post-Encroachment Time (PET). Results demonstrate that at a mainline design speed of 120 km/h, a 270-m acceleration lane reduces traffic conflicts by 47.6% compared to the conventional 230-m benchmark, while a 161-m deceleration lane achieves a 32.8% reduction. This research provides actionable design recommendations for optimizing speed change lane lengths on two-way ten-lane expressways, ultimately enhancing road capacity, safety, and traffic resource allocation efficiency.