(Photo © J. Maus/BikePortland)
Update: See below for a few other examples of graphics that try to answer this question.
There's an interesting, useful bit of transportation wonkery in The Bicycle Transportation Alliance's "Blueprint for World-Class Cycling" that came out this week: a visual guide to which sort of streets should get which sort of bike infrastructure.
This is obviously a complicated question, and it's not something that's ever going to be summarized by a single chart. But the question arises constantly. Last week, in a moment of heat, two Swan Island transportation advocates said the city would be better off without bike lanes near a crash-prone intersection of Interstate Avenue. Up in Vancouver, Wash., there's a lively debate right now about whether sharrows are appropriate on a 35 mph four-lane street.
Here's what the BTA's new document has to say about the issue: