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To the Editor: 

Interpreting the population density numbers being bandied about is a challenge. Let me address impacts on transit. As most of you know, that has been my field for 30 years. 

Average density for cities or neighborhoods is irrelevant to transit ridership. What counts is how many people and what activity is within walking distance of stops or stations. The rule of thumb is that people walk up to five minutes to a bus stop. On flat land, that is around a quarter mile. Both numbers stem from research on how far passengers actually walk. I have done some of the research. Our results duplicated those of other investigators. Historically, people have tended to walk further to rail transit stations. 

It is important to understand what the quarter mile means. It means that few passengers walk further. Beyond a quarter mile, you are looking at the tail of the distribution. Most passengers come from less than that. Since they come from all directions, imagine concentric circles, the market penetration (riders per unit of area) tails off even more rapidly with distance from the stop. 

Residences and activities have to be clustered really densely around stops if we expect many people to use transit. “Around” means just that. The high-density node should extend a block or two on either side of the arterial as well as along it. 

If we seek to shift future travelers from automobiles to transit, zoning must encourage high density at stops, not inhibit it. 

Robert R. Piper 

Former Director of Transportation 

Berkeley