Q: There’s a freeway exit I sometimes take that ends in a roundabout. The speed limit for the roundabout is 10 mph. When traffic is heavy, it is very risky to enter the roundabout because people do not slow down. Would speed bumps prior to the roundabout get people to slow down or would that make it worse?
A: Before we get into roundabouts, let’s talk about speed limits. We tend to think of a speed limit as the sign with black numbers on a white background. That’s part of it, but the first paragraph of the speed limit law requires drivers to travel at a reasonable speed for conditions, be aware of potential and actual hazards, and limit speeds to avoid a collision. Sometimes the legal speed is lower than what’s posted on a speed limit sign.
Take roundabouts, for example. You may have noticed on approach to a roundabout a yellow sign with the roundabout symbol, and below it a yellow placard with a speed on it (usually 15 or 20 mph.) That’s not a speed limit; it’s an advisory speed—a recommended speed based on highway design. You’ll often find advisory speeds paired with a warning sign alerting you to an upcoming change in “horizontal alignment,” which is traffic engineer speak for a curve in the road. You can tell a warning sign from a regulatory sign by the background color (yellow or white, respectively).
I’m familiar with a stretch of highway that has a 50-mph speed limit and roundabouts at the intersections. If someone were to try to drive at the posted speed limit through the roundabout, instead of driving closer to the advisory speed of 15 mph, there’s a good chance they wouldn’t make it through without hitting something or losing control. Even though they haven’t exceeded the posted speed limit, their speed isn’t reasonable for the conditions and they’ve ignored the warnings about changing road design. They’ve violated the first part of the speed law.
When you said the speed limit for the roundabout is 10 mph, I was skeptical, even as an advisory speed. The Department of Transportation Design Manual specifies that roundabouts be designed for speeds of 15 to 25 mph. But I did some searching and, while uncommon, there are roundabouts with 10 mph advisory speeds.
It’s possible though, in favorable conditions, to drive safely above an advisory speed. Advisory speeds are set conservatively, with consideration for the comparatively slower cornering of trucks and for drivers who are unfamiliar with the road. Given a design speed of 15 to 25 mph, it may be safe, absent any additional factors, to drive a car through the roundabout at 20 mph, even though it has a 10-mph advisory speed.
You’re not likely to see every driver follow the 10-mph advisory speed in your roundabout, but if drivers are coming into the roundabout faster than the design speed, there are options to slow them down. The best strategy is to design roads that naturally encourage driving at safe speeds, and roundabouts are usually pretty good at that. Speed bumps wouldn’t be the right choice, as they’re intended for very low speeds, like five mph. But raised crosswalks are an approved strategy for encouraging slow vehicle speeds in roundabouts. They’re like a speed bump, but as wide as a crosswalk with gentler slopes, allowing for speeds consistent with roundabout design.
And to all those in a hurry in a roundabout, it’s better to drive slow and wish you were going faster than to drive fast and wish you were going slower.