In road traffic networks, large traffic volumes may lead to extreme delays. These severe delays are caused by the fact that, whenever the maximum capacity of a road is approached, speeds drop rapidly. Therefore, the focus in this paper is on real-time control of traffic input rates, thereby aiming to prevent such detrimental capacity drops. To account for the fact that, by the heterogeneity within and between traffic streams, the available capacity of a road suffers from randomness, we introduce a stochastic flow model that describes the impact of traffic input streams on the available road capacities. Then, exploiting similarities with traffic control of telecommunication networks, in which the available bandwidth is a stochastic function of the input rate, and in which the use of effective bandwidths have proven an effective input rate control framework, we propose a similar traffic rate control policy based on the concept of effective bandwidths. This policy allows for increased waiting times at the access boundaries of the network, so as to limit the probability of large delays within the network. Numerical examples show that, by applying such a control policy capacity violations are indeed rare, and that the increased waiting at the boundaries of the network is of much smaller scale, compared to uncontrolled delays in the network.