Abstract: Modern smartphones differ vastly from classic cellphones with enormous capabilities, changing cellphones from making phone calls and short messages to pocket-sized computers with full internet connectivity. The downside of modern phone evolution is changing drivers' behavior and phone usage addiction. Several researchers have investigated the safety implications of phone usage while driving on accidents risk. Nevertheless, the effect of phone usage on the performance of traffic facilities did not receive equal attention.Hence, this research aims to study the impact of smartphone usage on traffic signal operations. The parameters considered in the study were drivers' activity while waiting at the signal, startup lost time, and saturation flow rate. The study team covered 25 intersections in the study area, observed the activities of over 6800 drivers while waiting at the signal, and recorded their corresponding discharge headways. The study team then supplemented field data with a behavioral survey questionnaire for drivers living in the study area to interpret field observations. Finally, the collected data were processed and analyzed, resulting in mathematical models for the relationship between the reduction in saturation flow rate and the red time interval.The team found two primary conclusions. The study's first finding was that the traditional concept for estimating startup lost time adopted by the highway capacity manual (HCM) appears inaccurate. The field results revealed that the lost times caused by other distracted drivers upstream in the queue were comparable to the estimated startup lost time following the HCM method for estimating startup lost time. The second finding was that the reduction in saturation flow rate at the traffic signal is an exponential function of the red time interval, reducing the overall capacity for the intersection and increasing the average delay. Finally, the authors suggest mitigating the problem and making recommendations for future research.