A Model to Predict Benefits of Priority Lanes on Freeways

An interactive computer-based model is described which permits a reasonable assessment of the decrease in the number of vehicle miles driven when one or several lanes of a freeway are devoted to the exclusive use of “carpools”: vehicles carrying at least a specified number of people. The vehicle miles driven are obtained by finding the equilibrium distribution of pool sizes as a function of distance driven, on both regular and priority lanes. A simple but believable economic model incorporating the cost of time and the “utility” of having a vehicle for one’s exclusive use is the basis for determining what individuals will do in their self-interest. The economic model is partly verified using existing pooling data without priority lanes. The idea of the priority lane or lanes is, of course, that they move faster than the regular lanes. However, an attractive feature of the priority lane concept is that, in typical cases, traffic flow is actually improved even for those who choose not to pool. Simple runs of the model show that in some cases it can be better for all concerned if two lanes are reserved for carpools rather than one lane, and if vehicles with two occupants are allowed to use the priority lanes rather than requiring three occupants. A 12.5-mi stretch of the Santa Monica Freeway in the Los Angeles area is used as a computational example for the theory. Further refinements to the model are suggested.