The heavy volume of vehicles arriving at toll plazas causes traffic congestion that contributes to longer travel time in toll roads. Utilizing the M/G/1 system under the queuing theory, a nonlinear programming model is developed to identify the appropriate speed limit to be imposed on the freeway of a toll road. The optimal speed limit minimizes the travel time of vehicles in the toll road while it maintains the queue length of vehicles waiting for payment within an arbitrarily set value. A formula solving for the optimal speed limit of a toll plaza operating a one type of tollbooth is developed by simplifying the constraints and observing the behavior of the objective function. The formula is tested using available traffic data.