OPTIMIZING THE TRAFFIC SYSTEM
In an earlier age the big concern of traffic engineers was simply building the physical transportation infrastructure. In the last several decades interest has grown in optimizing the performance of the existing system—of getting more performance from a given number of lane miles. The two terms here, somewhat overlapping, are Transportation System Management (TSM) and Transportation Demand Management (TDM).18 Both refer primarily to nonstructural system improvements.
The TSM category includes various tolling systems (see the next section of this chapter), signal coordination and synchronization, incident management (meaning clearing traffic blockages as expeditiously as possible) and entrance ramp metering to reduce congested flow on main arteries. It may also include replacing some traditional intersections with roundabouts (traffic circles), as these increase intersection capacity and reduce collisions. Another technique is providing motorists with real-time information on traffic conditions and detouring when advisable. TSM techniques may also include various parking controls to prevent obstruction of traffic flow, providing park-and-ride facilities, and taking other steps to reduce traffic flow in critical areas. Note that these later items could be classified just as well as TDM techniques.
Some TDM techniques include the promotion of van pooling and carpooling, encouraging staggered work hours to flatten out the morning and evening traffic peaks, encouraging telecommuting, using parking fees to encourage people to use other modes than driving solo, encouraging cycling and other nonautomotive modes, and promoting public transportation. In a longer view, there is obviously a nexus between TDM and urban design practices that mix land uses so as to reduce the average trip distance.