Back-Seat Driver

Who decides speed limits? Here’s the surprising answer

Cities are required to conduct speed surveys on major streets to determine the appropriate speed limit.
Cities are required to conduct speed surveys on major streets to determine the appropriate speed limit. Sacramento Bee file

Who determines the speed limit on streets you drive daily? In a lot of cases, it’s ... you.

Lincoln resident Jerry Mohlenbrok brought the question up last week. His City Council recently approved several speed limit increases on major streets in the growing Placer County city. It left Mohlenbrok perplexed. The streets are getting more crowded. Why is the city increasing speed limits, and how does it decide?

The answer is complex. Like it or not, drivers often get to vote with their gas pedal foot.

State law requires cities to measure the speeds that drivers travel on certain – but notably not all – streets, and use the results to adjust speed limits. The streets where local officials are required to do “speed surveys” are known in the traffic engineer lingo as collectors and arterials.

An arterial is a big street, typically four lanes, such as Joiner Parkway in Lincoln, Watt Avenue in Sacramento County, or Folsom Boulevard in Rancho Cordova. Collector streets are typically two-lane streets that take traffic from a residential area to an arterial.

When traffic engineers do their surveys, they are told to disregard the speeds of the fastest 15 percent of drivers on that street during free-flow traffic conditions. That’s because the law figures those people are probably driving too fast.

Some drivers are going too slow, as well. So the law says: Look at the speed that drivers near the 85th percentile are going, and set the limit there. That speed, engineers point out, is one standard deviation about the average speed for all cars on the road. Put another way, it’s a speed that is considered reasonable, given what’s actually happening each day on that street.

So, “the speed limits are really set by the drivers, not by the city or city council,” Lincoln city engineer Ray Leftwich says. “What speed does the driver feel comfortable traveling? That is what the road can accommodate.”

“Wait!” you say. You’re a good driver, but the drivers around you, not so good. Fortunately, many speed limits are not determined by surveys. Smaller residential streets, business district streets or streets that run past schools typically are automatically given a 25 mph limit. Also, freeway speeds are set by state officials, not by drivers.

City officials conducting speed surveys can round up or down in 5-mph increments. If a certain street’s 85th-percentile drivers are going 34 mph, traffic officials likely will set the limit at 35, but they can round down to 30 if they determine there is a particular safety issue. Lincoln, as a practice, always rounds down, Leftwich said.

Also, if crashes are up, cities likely will add a traffic signal, warning signage, or other traffic-calming devices to slow cars.

Cities generally don’t change speed limits often. Sacramento city hasn’t raised any speed limits in eight years, but has lowered a few, city officials said.

In Lincoln, the city’s recent survey of 25 streets led to speed limit increases on six. Leftwich said some of those streets had construction activity the last time they were surveyed, slowing traffic. The city did drop the speed limit on one street, a part of Lincoln Boulevard, from 55 mph to 50 mph.