Thumbs Down to Planning Rules of Thumb

Joe Cortright says rules of thumb used in transportation and land use planning need to be reevaluated. "One of the problems with rules of thumb (or the more academic term, 'heuristics') is that while they may work well in many cases, they may work very poorly in others – and they may be subject to important cognitive biases that lead us to make bad decisions."

For example, the idea that levels of service and maximum vehicle throughput should guide street design has led to streets that are too wide and do not accommodate other modes as well as an inefficient hierarchy of roadways. Assumptions about parking requirements and trip generation estimates for particular land uses have also resulted in problematic outcomes, argues Cortright.

He offers a different set of heuristics: closer is better, slower is safer, and sharing is efficient. He also believes accessibility, not mobility, should be the priority. "Many transportation heuristics emphasize speed: how do we make things move faster. But what we really care about is getting to (or being at) our destinations, not rapidly traveling among them. Speed should be secondary to choice."