Defining Normal in a Parking Garage

Here's another great passage from Tom Vanderbilt's Traffic. Vanderbilt is explaining why traffic behaves differently in different places, and refers to the work of psychologist Robert Cialdini to explain how social norms work:

"In one study, handbills were placed on the windshields of cars in a parking garage; the garage was sometimes clean and sometimes filled with litter. In various trials, a nearby 'confederate' either littered or simply walked through the garage. They did this when the garage was filled with litter and when it was clean. The researchers found that the subjects, upon arriving at their cars, were less likely to litter when the garage was clean. They also found that subjects were more likely to litter when they observed someone else littering, but only if the garage was already dirty.

"What was going on? Cialdini argues there are two different norms at work: an 'injunctive norm,' or the idea of what people should do (the 'ought' norm), and a 'descriptive norm,' or what people actually do (the 'is' norm). While injunctive norms can have an impact, it was the descriptive norm that was clearly guiding behavior here: People littered if it seemed like most other people did. If only one person was seen littering in a clean garage, people were less likely to litter -- perhaps because the other's act was so clearly violating the injunctive norm. This is why so many public-service advertising campaigns fall on deaf ears, Cialdini and others have suggested." (pages 228-229)

The correlation to behavior in traffic is obvious... who doesn't feel like it's pretty much okay to go the speed of traffic on a highway, even when that speed is five or 10 miles over the limit?