Slowing my roll

With the right mindset, speed-limiting technology can be liberating

August 13, 2024

By Ian Reagan
Senior Research Scientist, IIHS

The first time I got behind the wheel of a loaner SUV kitted out with an aftermarket anti-speeding device this spring, I was nervous.

As a safety researcher at IIHS, I've studied the benefits of intelligent speed assistance (ISA), which can detect the speed limit and restrict the power output of your engine to keep you from exceeding it. I've more or less argued it should be required in all new vehicles to help stem a speeding epidemic that kills thousands of Americans every year. But I'd never driven with it before and secretly worried I might hate it. Was I about to put myself in an awkward position?

The 35-mile commute from my home in Annapolis, Maryland, to my office in Arlington, Virginia, runs smack through Washington, D.C. If I'm lucky, it takes 45 minutes, but it can often take an hour and a half. The drive through Maryland is mostly expressway driving with a 65 mph limit and moderate traffic. It's the part that's inside the Capital Beltway - the 64-mile loop around the District of Columbia and its inner suburbs - where things can get hairy. Even a minor incident like a cardboard box falling off a truck can result in mile after mile of soul-sucking, stop-and-go paralysis.

I worried the ISA device would rarely activate inside the beltway due to such congestion, robbing me of the opportunity to see how it managed the multiple speed limit zones of the District. Outside the beltway, I was afraid it would make me feel like a rolling roadblock, as faster-moving traffic jockeyed to get around me.

Both fears turned out to be exaggerated - and somewhat beside the point.

A drive from Annapolis, Maryland, to Arlington, Virginia - similar to the author's commute - involves a range of road types and speed limits, making it an ideal route to test out intelligent speed assistance.

ISA systems have been around for several decades. Using GPS devices with speed limit maps, onboard sensors or cameras that "read" signs or both, they can identify the speed limit in the vehicle's location and detect when the driver exceeds it, but they vary in their response. Unlike the unit I used, which worked by restricting the engine's output, some systems only provide an audible or visual warning or make the accelerator harder to press. As of July 1, new vehicles sold in Europe are required to have ISA that at least provides a warning, and the California legislature is currently considering a similar mandate for vehicles sold in the state.

Even without a legal requirement, ISA systems of various types are starting to appear as options on new vehicles in the U.S., and companies such as MAGTEC and Sturdy sell aftermarket ISA systems, primarily targeting fleet operators. Navigation apps also include speed warnings if the user chooses.

The aftermarket device that I tested determined the speed limit solely by GPS connected to a speed limit map. Provided by Sturdy and installed on a 2023 Hyundai Tucson, it worked a little like cruise control, only in reverse. When it kicked in, I could still press the accelerator, but the vehicle's speed wouldn't increase. It also allowed me some choice about when that happened - when I matched the speed limit, when I was 5 mph over or when I was 10 mph over. I tested the device in all three settings on the expressway segments of my commute and used the speed limit threshold on surface roads.

By and large, I found that the system accurately detected the speed limits on my route. However, a colleague who tried it out was frustrated by inaccuracies where he lives. A temporary override button allowed him to bypass the system and speed up when that happened.

Sturdy informed us that such discrepancies are rare - and updated their maps based on our feedback. I expect the number of inaccuracies to decrease rapidly as ISA use becomes more widespread, much the way navigation apps have improved over time.

Driving with the flow of traffic, I had ample opportunity to experience the system kicking in and keeping me from accelerating. To my surprise, it even happened inside the beltway, on both expressways and city streets, and I found that my impressions about the system depended on the roadway environment in a more complex way than I anticipated.

For one thing, I was surprised how useful it was on urban streets. Our Arlington office is in a densely populated area of high-rise office buildings, condominiums, shops and restaurants. The speed limit is generally 25 mph. I wouldn't have predicted this going in, but the ISA system kicked in to restrict my speed repeatedly in this neighborhood, where I think of myself as driving slowly and carefully because of all the pedestrians and bicyclists. Mostly, it happened when I was accelerating away from an intersection after I had been stopped at a red light.

It's in this kind of neighborhood where ISA could have the biggest safety impact. Higher speeds greatly increase the risk of severe injury or fatality for pedestrians. On average, there's a 25% chance that a pedestrian struck by a vehicle going 25 mph will sustain a severe injury. If the vehicle is traveling just a little faster, at 33 mph, that number rises to 50%.

Of course, even when there are no pedestrians around, controlling speed is crucial for safety. Higher speeds reduce the time the driver has to react and increase the distance it takes the vehicle to stop, while a small uptick in the impact speed has a dramatic effect on crash forces. For example, when impact speed increases from 40 to 60 mph (a 50% increase), the energy that needs to be managed increases by 125%.

I often hear drivers saying they're sometimes surprised to discover they're speeding due to the smooth, quiet ride of today's vehicles. These anecdotes are typically about highway driving, but the same masking factor could contribute to speeding on lower-speed roads. In this setting, I felt the ISA system wasn't stifling a desire to drive above the limit but preventing me from doing so by accident.

This was especially true during the D.C. portion of my commute, which includes a lot of limited-access roads (those with on and off ramps) that have lower speed limits than you might expect because of the generally high volume of traffic.

The first time I drove home in sparse traffic late at night, the system startled me by slowing me down when I was crossing D.C.'s 14th Street Bridge. After the fact, I realized that the ISA device had rightly detected a reduction in the speed limit to 40 mph from 55 mph, a change that is poorly marked with a single, small sign on the right shoulder. As far as I know, none of the over 200 speed safety cameras currently in use in the District is deployed on the bridge, but the discovery provided a good reminder that the ISA device was not only helping me drive safely, it was also helping me avoid the expense and hassle of a ticket.

Another speed limit change on my commute occurs just outside the beltway. Driving toward the District, the road's limit goes from 65 mph to 55. Heading home, the reverse is true. When the speed limit dropped at this spot, the ISA device responded much faster than the stream of traffic. Going the other way, the device held me to the lower limit until we reached the 65 mph sign, while other drivers sped up in anticipation of the change. I felt like I was aggravating other drivers by slowing much earlier and accelerating much later than everyone else.

In response, I tried to shift my mindset. Instead of worrying about other drivers, I kept the SUV in the middle or right lane of the expressway. Faster moving traffic began passing me on the left, and I began to see how driving with ISA could also lead to a less stressful commute: I wasn't worrying about getting a ticket, and I wasn't fighting for position in a pointless race that might not even wind up shaving any time off my journey.

Public link

Please use the above public link if you want to share this noodl on another website.