How to deal with the probabilistic nature of traffic?

Traffic is probabilistic by its nature. This is due to variances in the traffic itself, such as the driver population, vehicle types, etc. But it is also due to the circumstances, like weather conditons and incidents. The question is how to deal with these variances and how to take into account the inherent variable nature of daily traffic congestion on the motorway network, when considering taking traffic management measures to alleviate this traffic congestion. Will this change the way in which existing traffic management measures are applied? Or are new measures needed to cope with this problem?