Chapter 34: Traffic Assignment Methods

Traffic assignment plays a key role in the traditional transportation planning procedure, which includes these four steps: trip generation, trip distribution, modal split, and traffic assignment. The purpose of traffic assignment is to distribute or load traffic on to the network according to a certain route choice principle. Over the years, refinements have been introduced to each of these four steps. Boyce (2002) provides some historical perspectives on this procedure and points to possible improvement directions. Nonetheless, this four-step procedure forms the platform for many existing transportation planning models.

Historically, in the absence of exact solution methods, heuristics were developed to produce reasonable answers to the traffic assignment problem. One such example is the land use transport optimization (LUTO) model developed in the early 1980s for strategic land use and urban transportation planning in Hong Kong. This approach was first developed in the UK (Hall et al., 1980; Willumsen et al., 1993), and was subsequently refined and deployed in Hong Kong (Choi, 1986). LUTO developed a stochastic multi-path network loading heuristic for the traffic assignment problem.