Author: Dr. Jean-Paul Rodrigue

Transportation seeks to minimize the effort of moving passengers and freight between locations. A component of this effort involves route selection.

1. Route Selection

Human beings are natural effort minimizers, notably when it involves moving around. When given the opportunity, they will always try to choose the shortest path to go from one place to another. This behavior can easily be observed from pedestrians. When possible, a pedestrian will walk over a lawn, zigzag by cars in a parking lot, or cross a street sideways between intersections if the route selected enables to reach faster a destination.

Transportation, as an economic activity, replicates this process of minimization, notably by trying to minimize the friction of distance between locations. Shorter times and lower costs are looked upon by all transport users from individuals managing their own mobility to multinational corporations managing complex supply chains. For an individual, it is often only a matter of convenience, but for a corporation it is of strategic importance as a direct monetary cost is involved. Under such circumstances, numerous methods have been developed to deal with the complex issue of route selection. One such classic application is the “traveling salesperson” problem, where a shortest route has to be selected from a set of possible paths.

Route selection has two major dimensions:

• Construction. Involves activities related to the setting of transport networks such as road and rail construction where a physical path has to be traced. Among the basic considerations are factors such as distance and topography.
• Operation. Concerns the management of flows in a network. This is the most common route selection activity since it considers routes as fixed entities and seeks an optimal path considering existing constraints.

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2. Evaluating the Route Selection Process

The choice of linking a location to another, and more importantly, the path selected is part of a route selection process which respects a set of constraints. Although route selection varies by mode, the underlying principles remain similar; in its most simple form, a route selection process (R) tries to respect these general constraints:

R = f(min C : max E)

Route selection tries to find or use a path minimizing costs (C) and maximizing efficiency (E). There are obviously two major dimensions in this function:

• Cost minimization. A good route selection should minimize the overall costs of the transport system. This implies construction as well as operating costs. The most direct route is not necessarily the least expensive, notably if rugged terrain is concerned, but most of the time a direct route gets selected. It also implies that route selection must be the least damageable to the environment, if environmental consequences are considered.
• Efficiency maximization. A route must support economic activities by providing a level of accessibility, thus fulfilling the needs of regional development. Even if a route is longer and thus more expensive to build and operate, it might provide better services for an area. Its efficiency is thus increased at the expense of higher costs. In numerous instances, roads were constructed more for political reasons then for meeting economic considerations.

Route selection is consequently a compromise between the cost of a transport service and its efficiency. Sometimes, there are no compromises as the most direct route is the most efficient one. At other times, a compromise is very difficult to establish as cost and efficiency are inversely proportional.

3. Traffic Assignment

Under construction.

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Bibliography

• Cambridge Systematics (2019) Quick Response Freight Methods, USDOT, Federal Highway Administration, Office of Planning and Environment Technical Support Services for Planning Research.