DO NEW HIGHWAYS GENERATE TRAFFIC

DO N E W HIGHWAYS GENERATE TRAFFIC? Some people seem to think there's a universal Law of Traffic Congestion ruling that traffic expands to fill the space available to it. T h u s , they believe it's impossible to relieve highway conges- tion by building more roads. Conventional wisdom aside, however, we simply don't know whether new highway capacity affects travel behavior and, hence, traffic volumes. Advocates and opponents of new highway con- struction have debated this point for many construction projects in recent years—without the hard proof necessary to enlighten their debates. Established planning procedures assume that most extra traf- fic on an enlarged road is reassigned from other parts of the net- work. Revisionists claim there are also sizable numbers of new BY M A R K HANSEN trips and net increases in vehicle miles of travel ( V M T ) . Thus advo- cates envision a fixed number of vehicles operating in improved traffic conditions, while opponents foresee more vehicles in con- ditions not much better, or perhaps even worse, than before. Mark Hansen is assistant professor engineering of civil at the University Berkeley, S S and CA environmental Everyone recognizes that road construction, particularly in of Ca I ifo mi a, urban areas, is very costly. Even if traffic inducement is omitted from the project analysis, there are difficult trade-offs between the