Susan Handy

The distinction between mobility and accessibility is what Handy is cited for, and it changes the criterion by which a city is judged. Mobility measures how much and how fast you travel; accessibility measures how many useful destinations you can reach in a reasonable time. A district with a motorway and no shops has high mobility and low accessibility; a dense walkable district has the opposite.

The practical consequence is large. If you measure mobility, the answer to every problem is another lane. If you measure accessibility, bringing destinations closer, mixing uses and improving the pavement compete on equal terms with widening the road, and usually cost less. Handy has spent thirty years supplying the empirical evidence for that comparison.

Her other line is methodological and much debated: the problem of residential self-selection. When we observe that people living in walkable neighbourhoods walk more, we cannot tell whether the neighbourhood produces the behaviour or whether those who already wanted to walk chose that neighbourhood. Handy designed the studies that separate the two effects, and found that the built environment does matter, though less than earlier work suggested.

An engineer and professor at the University of California, Davis, Handy directs the National Center for Sustainable Transportation there and advised the State of California on the reform that replaced traffic level of service with vehicle miles travelled as the environmental review criterion, one of the most important regulatory changes in recent American planning.