Staff Columnist

Speed limits don't do what you think they do

A recent case in Iowa City shows the tension between central planning and spontaneous order

Traffic is reduced to one lane in each direction on Dubuque Street as work continues the Iowa City Gateway Project in Iowa City on Tuesday, Aug. 29, 2017. (Stephen Mally/The Gazette)
Traffic is reduced to one lane in each direction on Dubuque Street as work continues the Iowa City Gateway Project in Iowa City on Tuesday, Aug. 29, 2017. (Stephen Mally/The Gazette)

Among all the tools governments have to regulate personal behavior, speed limits must be one of the most misunderstood.

I had long assumed driving speeds were dictated to a significant extent by the posted speed limit. In well-planned cities, however, it’s the exact opposite — posted speed limits are determined by real observed driving speeds.

Iowa City recently increased the limit on a short span of Dubuque Street from 25 to 35 miles per hour. It’s a heavily traveled access point to the community, connecting Interstate 80 to downtown Iowa City and the University of Iowa campus.

Governments can set artificially low speed limits, but police do not have the ability to stop and write tickets to everyone who breaks the speed limit.

-

That street had been 35, until the portion around Mayflower Residence Hall was reduced during construction of the “Gateway Project,” which raised Dubuque Street and supplied a new Park Road bridge to ward off roadway flooding from frequent seasonal flooding from the Iowa River.

At a work session this month, city transportation planners got approval from City Council members on their recommendation to restore the 35 mph limit approximately between Ridge Road and Kimball Road.

Officials sometimes use the 85th percentile of speed to set limits — the observed speed that 85 percent of vehicles are traveling at or below. On this span of Dubuque Street, the 85th percentile speed ranges from about 36 to more than 41 mph, depending on which lane, according to the city.

The average speeds range from 32 to 36 mph. In other words, most people were breaking the speed limit, and a significant portion were going more than 10 mph over.

ARTICLE CONTINUES BELOW ADVERTISEMENT

Meanwhile, city staffers report there has been no significant change in the number or severity of collisions in the affected area in the past several years, during construction and the reduction of the speed limit. Importantly, there have been zero reported pedestrian or bicycle collisions, and more pedestrians have been observed using the controlled intersection to the south.

Even if you’re not the type of person who spends idle car time thinking about traffic control, you probably have noticed some roads “feel” like the speed limit is too high or too low based on factors like the width of the road, visibility, the number of upcoming intersections or access points and proximity to pedestrians.

If the Iowa City Council had insisted on sticking to 25 mph, it would have required more enforcement or new physical measures to calm traffic.

Governments can set artificially low speed limits, but police do not have the ability to stop and write tickets to everyone who breaks the speed limit, nor should we want to bear the incursions on our time and privacy that total enforcement would require.

As our neighbors in Cedar Rapids and elsewhere have shown, even limited uses of traffic cameras are fraught with political peril, not to mention the enormous surveillance apparatus it would take to automate speed enforcement on a large scale.

Speed limits are a fascinating study in the tension between central planning and spontaneous order. Governments can impose a wide range of laws and regulations, but if they can’t be workably enforced, they don’t much matter.

Comments: (319) 339-3156; adam.sullivan@thegazette.com

Give us feedback

We value your trust and work hard to provide fair, accurate coverage. If you have found an error or omission in our reporting, tell us here.

Or if you have a story idea we should look into? Tell us here.

Give us feedback

We value your trust and work hard to provide fair, accurate coverage. If you have found an error or omission in our reporting, tell us here.

Or if you have a story idea we should look into? Tell us here.