An integrated decision-support framework for spatial siting of midblock flyovers based on traffic operations, road safety and transport economic appraisal

High crash rates and recurrent congestion are common on highway corridors where industrial access, U-turn movements, and high-speed through traffic interact within constrained roadway environments. Although midblock flyovers can improve corridor performance, their spatial siting has received considerably less attention than the broader decision to provide grade separation. This study develops an integrated decision-support framework for evaluating alternative flyover locations through traffic operations, historical crash evidence, operational and environmental costs, traffic interactions, and economic appraisal. The framework was applied to a freight-intensive industrial corridor in southern Thailand, where an at-grade baseline and four alternative flyover locations were evaluated under three traffic-demand conditions. Lane-based operational modelling was integrated with an exposure-based crash assessment, monetized safety benefits, and localized traffic-interaction analysis. All grade-separated alternatives substantially improved corridor performance, reducing average delay to below 6 s/veh while decreasing queues, fuel consumption, emissions, and operational and environmental costs. Benefit–cost ratios of approximately 1.56–1.57 demonstrated the economic justification for grade separation. Spatial siting nevertheless influenced localized operations and residual traffic interactions. Locations farther from the industrial access exhibited lower localized delay and shorter queues while reducing the concentration of crossing interactions near the industrial access. Among the otherwise comparable alternatives, farther-upstream siting may also provide greater functional flexibility for vehicles requiring post-flyover access to the industrial facility. These findings distinguish investment justification from spatial evaluation: aggregate operational and economic measures establish whether grade separation is warranted, whereas localized traffic interactions and industrial-access functionality provide the basis for determining where it should be located.