The concept of allocation of departure time slots is applied to alleviate and analyze road transport congestion. Demand is redistributed over departure time slots at on-ramps to analyze its relationship with the congestion in the network with minimized system travel time and optimized network utilization. The objective is to minimize the costs of early and late departure associated with any differences between the cumulative desired and optimum demand, with network capacity as a constraint. The genetic algorithm, a global-searching algorithm, is used to calculate the objective function. Then the consequences of the optimum demand for the traffic states are assessed through network simulation. The application of the approach of the A15 motorway in the Netherlands shows that early departure occurs for short journeys with low demand and late departure occurs mostly for long journeys with high demand. Moreover, the traffic state improves greatly with 7.0% reduction in total travel time and 8.9% reduction in congestion of temporal-spatial combinations. Authorities may use these results to take measures or make design policies that promote time-slot allocation, such as providing rewards for late or early departure outside the peak period.