Management of road intersections plays a key role in dealing with urban traffic congestion. The study aimed to design a traffic signal light system at the Romulo-Highway and Tibag-San Isidro Road, a major intersection in Tarlac City, Philippines. Using LocalSim, a microsimulation software specifically developed for Philippine local conditions, the study simulated actual and optimal traffic signal light designs for morning and afternoon peak, considering the traffic conditions after the lockdown caused by the COVID-19 pandemic. The average traffic speeds observed were 16.91 kph and 26.62 kph in the morning and afternoon, respectively. Through simulation, the optimized signal light designs obtained were at 64 s per cycle in the morning and 42 s in the afternoon. This led to a decrease in the average vehicular flow rate at the intersection. In the morning, the flow rate from Tarlac City decreased from 1191 vph to 909 vph, while the traffic flow towards San Isidro decreased from 267 vph to 200 vph. On the other hand, the afternoon flow rate from Tarlac City decreased from 1229 vph to 1071 vph, while the flow rate from Sta. Ignacia decreased from 1288 vph to 1189 vph. Consequently, some of the intersection segments improved their level of service (LOS) from C to B or B to A. Moreover, the simulation indicated an increase in travel time of an additional 99 s in the morning and 63 s in the afternoon. Furthermore, the traffic speed decreased to 11.71 kph in the morning and 20.08 kph in the afternoon. In conclusion, it was found that installing a traffic signal light system at the intersection of Romulo-Highway and Tibag-San Isidro Road generally resulted in a longer travel time and a slower travel speed but a better LOS and more stable vehicular flow.