Some commonly used “rules of thumb” produce very bad results
We all know and use rules of thumb. They’re handy for simplifying otherwise difficult problems and quickly making reasonably prudent decisions. We know that we should measure twice and cut once, that a stitch in time saves nine, and that we should allow a little extra following distance when the roads are slick. There’s a second, and related metaphor involving thumbs: “putting one’s thumb on the scale,” meaning to bias the results of a measurement by tipping the scale in one direction to achieve a desired outcome. That’s equally applicable to many of these rules of thumb: they exist and are crafted as they are to push decisions in a particular direction. That’s especially true for many commonly applied planning rules.
What purport to be “standards” in the worlds of transportation and land use are in many cases just elaborate rules of thumb. And while they might have made sense in some limited or original context, the cumulative effect of these rules is to tip the scales so that we have a transportation system which is by regulation, practice, and received wisdom, “all thumbs.”














