Orlando Sentinel columnist Beth Kassab quoted me in her column last weekend, "Square footage is actually a very poor determinant of parking demand." I gave Beth an example from Donald Shoup's High Cost of Free Parking, criticizing the Institute of Transportation Engineers' parking demand manual for fast food restaurants with drive-throughs:
Each dot on the matrix shows peak parking demand in relation to the gross square footage of the restaurant, according to various studies. Two restaurants, only 2,500 square feet in size, created demand for 42 and 43 parking spaces. Yet another restaurant, 5,500 square feet—more than twice their size—created peak demand for only 20 parking spaces. In other words, square footage poorly predicts parking demand. ITE even acknowledged this with its disclaimer, “Caution – Use Carefully -- Low R2.” However, ITE also drew a line on the matrix (a “fitted curve”), recommending that 2,500 square foot restaurants have no fewer than 24 parking spaces. The line gives the illusion of scientific precision when, in fact, there’s none. (If the studies showed reasonable predictability, the dots would congregate much, much closer to the line.) Cities and counties adopt what’s recommended on the line into their codes.