we can't drive 55

I stumbled across fascinating study today, commissioned by the U.S. Department of Transportation. The aim of the study:

The objectives of this research was to determine the effects of raising and lowering posted speed limits on driver behavior and accidents for non-limited access rural and urban highways. Speed and accident data were collected in 22 States at 100 sites before and after speed limits were altered. Before and after data were also collected simultaneously at comparison sites where speed limits were not changed to control for the time trends. Repeated measurements were made at 14 sites to examine short - and long-term effects of speed limit changes.

The conclusions were interesting:

  • Lowering speed limits by 5, 10, 15, or 20 mi/h (8, 16, 24, or 26 km/h) at the study sites had a minor effect on vehicle speeds. Posting lower speed limits does not decrease motorist’s speeds.
  • Raising speed limits by 5, 10, or 15 mi/h (8, 16, or 25 km/h) at the rural and urban sites had a minor effect on vehicle speeds. In other words, an increase in the posted speed limit did not create a corresponding increase in vehicle speeds.

The primary conclusion of this research is that the majority of motorist on the nonlimited access rural and urban highways examined in this study did not decrease or increase their speed as a result of either lowering or raising the posted speed limit by 4, 10, or 15 mi/h (8, 16, or 24 km/h). In other words, this nationwide study confirms the results of numerous other observational studies which found that the majority or motorist do not alter their speed to conform to speed limits they perceive as unreasonable for prevailing conditions.

There you have it: posted speed limits don’t really matter.

This reinforces what I’ve always suspected: within a certain range, people will drive at the speed that “feels” best for the road, regardless of the posted speed. (However, I do suspect that if you lowered the speed limit enough, the risk of being snagged for reckless driving increases such that people would actually slow down.) It seems like this has some interesting ramifications for strategic traffic management. If you want people to slow down, I’d theorize that the design and look of the road has more to do with how fast people drive than the posted speed limit.

Take, for example, the road my parents live on in west Nashville, Davidson Dr. This road has a posted speed limit of 30 mph. Despite this, most people drive, on average, no less than 40-45 mph – myself included. Why? It “feels” right. It’s a long stretch of road, with sections of wooded areas on either side. When there are houses, they are all separated by large, sloping half-acre lots of yard. The lanes are wide, and there are very few pedestrians. Every curve of the road is easily turned at 40-45 mph. The road gives little indication of higher speeds posing a danger to either the driver or anyone else.

This is a case where the posted speed limit just doesn’t match the “feel” of the road. In my opinion, the speed limit should be raised, or steps should be taken to make an effort to make the road match the desired speed. In the case of this road, it’s a true chicken/egg dilemma, but I’d wager that there would be more pedestrians on this road if people didn’t drive so fast – and if there were sidewalks. Nobody likes to live on a busy road, and sometimes it’s inevitable. But a busy road mustn’t always be a fast road.

What could they do to drop the speed on roads like this? Simple: sidewalks. Traffic light(s). Stop signs. narrower lanes. Lots of neighborhood organizations even go as far as to campaign for speed bumps on smaller neighborhood streets.

The downside is that these things are expensive, especially when compared to the cost of a few speed limit signs (not to mention the added revenue from the resulting tickets).

Given the expense involved, you might surmise that lower-income neighborhoods are less prone to get this sort of attention to detail. Based on my very limited sample set (my neighborhood), I’d guess this is accurate. My neighborhood is a low-middle income area (however, it is in the throes of gentrification). People drive entirely too fast on my street, despite some of the visual cues that you’d think would cue people to slow the %*(#$* down: a very narrow street, lots of pedestrians, and cars parked on either side of the street. The posted speed limit clearly isn’t making one bit of difference: namely because there would never be a cop car around to enforce it anyway. There rarely are on side-streets. I think a few more of these “cues” would go a long way in making the road safer: a few stop signs and sidewalks, for example.

But, Nashville is a city notorious for its poor urban street design. The common wisdom is that Nashville spends more on its highways than its schools. Freeways, maybe, but that sure isn’t reflected in the smaller streets. Sidwalks are unheard of, and drainage is notoriously awful (and dangerous, at times).

I think the city planners could do well to re-invest some of the revenue from poorly posted speed limits into smarter street design that would make our streets safer – and posted speed limits superfluous.