Impact Assessment of Signal Head Position on Start-Up Lost Time and Saturation Flow Rate at Urban Signalized Intersections

Signal head position can influence driver visibility and response at signalized intersections, but its effect on operational performance remains unclear under real-world conditions. This study examines the association between near-side and far-side signal head positions and two operational measures, start-up lost time and saturation flow rate, at selected urban signalized intersections in Dammam and Jubail, Saudi Arabia. Field data were collected during the evening peak period and analyzed to compare performance between the two cities and among individual intersections. The results showed that far-side signals in Jubail were associated with significantly lower start-up lost times than near-side signals in Dammam, with an average difference of approximately 0.60 s. However, the difference in saturation flow rate was not statistically significant, with average values of 1666 veh/h/lane in Dammam and 1590 veh/h/lane in Jubail. Significant variation was also observed among some intersections with the same signal-position configuration. These findings indicate that the observed differences in signal head position were associated with differences in initial driver response; however, the observed effects cannot be attributed to signal head position alone because the two signal configurations were examined in different urban contexts. The absence of a corresponding increase in saturation flow rate indicates that the observed reduction in start-up lost time should not be interpreted as evidence of higher lane capacity. Actual lane capacity was not directly calculated in this study and additionally depends on effective green time, cycle length, signal-control strategy, and other site-specific operational factors. Therefore, signal head placement should be evaluated as part of a broader intersection performance assessment rather than as an isolated design feature.