Optimal control of motorways by ramp metering, variable speed limits, and hard-shoulder running

Ramp metering, variable speed limits, and hard-shoulder running have been used for managing motorway traffic congestion. The majority of the research in the area of ramp metering strategies is concentrated on local traffic responsive algorithms, such as ALINEA. Recently, researchers have studied the effects of variable speed limits and hard-shoulder running applied on motorways. This paper presents an optimization framework for all these control strategies. The framework was developed on the basis of a macroscopic cell transmission model (CTM), which described traffic dynamics through a piecewise linear fundamental diagram. With the piecewise linear nature of the CTM, the authors formulated optimal control problems that would seek the optimal control policies for minimizing total delays on motorways. The optimal control problems were formulated as linear programming or mixed integer linear programming and were solved by using the IBM CPLEX solver. The performances of different control strategies were tested on real scenarios on the M25 motorway in England, where significant improvements were observed with proper implementation.