A connected traffic monitoring system comprises at least one Roadside Unit (RSU) and a traffic signal controller. The roadside unit is configured to transmit wireless signals, receive corresponding responses from a first Onboard Unit (OBU)-equipped vehicle and a second OBU-equipped vehicle and send data from the first OBU-equipped vehicle and the second OBU-equipped vehicle to the traffic signal controller. The traffic signal controller to calculate a distance between the first Onboard Unit (OBU)-equipped vehicle and the second OBU-equipped vehicle in a vehicle queue associated with a traffic light signal on an intersection, determine the queue length of the vehicle queue, determine whether the distance between the first OBU-equipped vehicle and the second OBU-equipped vehicle is greater than a vehicle length and if the distance is determined greater than the vehicle length, detect at least one non-OBU-equipped vehicle stopped in the vehicle queue behind the first OBU-equipped vehicle.