When Signal Timing Is Not Enough: Drone-Data-Driven Microsimulation for Geometric Redesign of a Congested Signalized Intersection

Abstract
Urban signalized intersections often experience recurrent congestion because the underlying cause of failure, whether insufficient signal allocation, inadequate geometry, or both, is not identified before intervention. This study evaluates the separate and combined effects of signal timing optimization and geometric redesign at the congested four-leg Tofaş Junction in Elazığ, Türkiye. Evening peak traffic demand was obtained from aerial drone footage and processed in DataFromSky to derive classified origin-destination matrices. An Aimsun Next microsimulation model was developed for the existing intersection, and SIDRA Intersection was used to identify an optimized signal plan. Six cycle lengths were tested for the existing layout and four for a redesigned layout. The redesign removed the large central island to recover roadway space, added dedicated left-turn and right-turn lanes on all approaches, introduced free-flow right-turn slip roads, and improved channelization through a western-approach median treatment. The existing 102 s signal plan produced an entry delay of 85.82 s/km, a mean queue length of 23.87 vehicles, and a stop time of 91.05 s/km. Signal retiming improved performance, with the 62 s cycle reducing entry delay to 50.01 s/km and mean queue length to 12.33 vehicles. However, the redesigned intersection performed best at a 77 s cycle, reducing entry and exit delays by 57%, mean queue length by 43%, stop time by 59%, and total stops by 34%. Entry speed increased by 55%, while traffic density decreased by 42%. Car fuel consumption was also 10.6% lower than in the existing baseline. The results demonstrate that signal optimization alone cannot fully overcome structural bottlenecks created by shared turning and through lanes. For severely constrained intersections, geometric correction should precede or accompany timing optimization to achieve reliable, sustainable operational improvements.