Imagine a stretch of road that gets lots of traffic and is heavily traveled during the morning and afternoon commute. The road is windy at parts, has numerous intersections, and some nearly blind turns that can become dangerous if traffic gets backed up. In the interest of safety, and after careful study, the city sets the speed limit at 35 mph.
This road gets extremely congested during the commute. If commuters did not consistently break the speed limit, traveling at 45, 55, 65 mph, the congestion would easily turn into a crawl with frequent stop-and-go activity. Most drivers justify this infraction by telling themselves that 35 mph is a stupid limit and the relative infrequency of traffic accidents (even at rush hour) proves it. The commuters appreciate the unspoken pact between them because, after all, if no such pact existed they would all spend a lot more time traveling to and from work. The benefit of breaking the speed limit outweighs the increased risk of winding up in an accident.
Now, if the speed limit were set democratically, the majority of these drivers would vote to increase the limit, despite the evidence which suggests that an increase would be detrimental to the safety of drivers. Insofar as this is not an unlikely example, I think it serves as a nice allegory for one of the weaknesses of democracy.