In July 2003 a new Road Code was approved by the Italian parliament. Among other changes, the law states that on three-lane motorways the right lane should not be reserved anymore to slow vehicles. The model developed in this paper casts doubts on the wisdom of such a change, suggesting that reserving a separate lane for slow vehicles is generally better, in terms of number of accidents and slow-downs, than treating all vehicles the same way. In addition, a value for the speed threshold below which vehicles are considered slow can be found, for which the old rule also performs better in terms of average speed. This conclusion has applications beyond the Italian case. Moreover, it is shown to be extremely robust to refinements of the main assumptions concerning driving attitudes and the stochastic arrival of accidents.
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