When I first got behind the wheel of a rental SUV equipped with an aftermarket speed limiter this spring, I was nervous.
As a safety researcher at the IIHS, I studied the benefits of Intelligent Speed Assist (ISA), which detects the speed limit and limits your engine’s power so you don’t exceed it. I more or less argued that it should be required in all new vehicles to counteract speeding that kills thousands of Americans every year. But I had never driven with such an assistant, and I secretly feared that I might hate it. Was I about to put myself in an uncomfortable position?
The 35-mile commute from my home in Annapolis, Maryland, to my office in Arlington, Virginia, passes through the middle of Washington, D.C. If I’m lucky, it takes 45 minutes, but often it can take an hour and a half. Most of the drive through Maryland is on freeways with a 65-mph speed limit and moderate traffic. It’s the part inside the Capital Beltway—the 64-mile loop around the District of Columbia and its inner suburbs—where things can get dicey. Even a minor incident like a box falling off a truck can result in miles of soul-sapping stop-and-go traffic.
I was worried that the ISA device would rarely activate due to the congestion inside the ring road, so I wouldn’t have a chance to test how it coped with the district’s numerous speed limit zones. Outside the ring road, I was worried that I would feel like a rolling roadblock as faster traffic tried to overtake me.
Both fears turned out to be exaggerated – and missed the point.
ISA systems have been around for several decades. Using GPS devices with speed limit maps, built-in sensors or cameras that “read” signs, or both, they can determine the speed limit at the vehicle’s location and detect when the driver exceeds it. However, they react differently. Unlike the device I used, which worked by limiting engine power, some systems only give an audible or visual warning, or make it more difficult to press the accelerator pedal. As of July 1, new vehicles sold in Europe must be equipped with an ISA that gives at least a warning, and California lawmakers are currently considering a similar requirement for vehicles sold in that state.
Even without a legal requirement, ISA systems of various types are increasingly being offered as an option on new vehicles in the United States. Companies such as MAGTEC and Sturdy sell aftermarket ISA systems aimed primarily at fleet operators. Navigation apps also include speed warnings on request.
The aftermarket device I tested determined the speed limit solely through GPS linked to a speed limit map. It was supplied by Sturdy and installed in a 2023 Hyundai Tucson. It worked a bit like cruise control, only in reverse. When it kicked in, I could still step on the accelerator, but the vehicle’s speed did not increase. It also gave me some choice as to when that happened – when I was obeying the speed limit, when I was going 5 mph too fast, or when I was going 10 mph too fast. I tested the device in all three settings on the highway portions of my commute and used the speed limit bump on country roads.
Overall, I found the system to be accurate at identifying speed limits on my route. However, a colleague who tried it out was frustrated by the inaccuracies where he lived. A temporary override button allowed him to bypass the system and speed up when that happened.
Sturdy told us that such discrepancies are rare – and updated its maps based on our feedback. I expect the number of inaccuracies to decrease quickly as ISA becomes more widely used, similar to how navigation apps have improved over time.
When driving in flowing traffic, I had plenty of opportunity to experience the system kicking in and preventing me from accelerating. To my surprise, this happened even inside the ring road, on both expressways and city streets, and I found that my impressions of the system depended on the road environment in more complex ways than I expected.
For one thing, I was surprised at how useful it was on city streets. Our office in Arlington is in a densely populated area with high-rise office buildings, condos, shops and restaurants. The speed limit is typically 25 mph. I wouldn’t have predicted this, but the ISA system kicked in to repeatedly limit my speed in this area, where I drive slowly and carefully because of all the pedestrians and cyclists. Most of the time it happened when I accelerated away from an intersection after stopping at a red light.
It is in such areas that ISA could have the greatest impact on safety. Higher speeds significantly increase the risk of serious injury or death to pedestrians. On average, the probability that a pedestrian struck by a vehicle traveling 25 miles per hour will be seriously injured is 25%. If the vehicle is traveling just a little faster, 33 miles per hour, that number rises to 50%.
Of course, controlling speed is critical to safety even when there are no pedestrians nearby. Higher speeds reduce the driver’s reaction time and increase the vehicle’s stopping distance, while a small increase in impact speed has a dramatic effect on impact forces. For example, if the impact speed increases from 40 to 60 mph (a 50% increase), the energy to be dealt with increases by 125%.
I often hear that drivers are sometimes surprised that they are speeding because today’s vehicles are so smooth and quiet. These anecdotes usually refer to driving on motorways, but the same masking factor could also lead to speeding on lower speed roads. In this situation, I felt that the ISA system was not suppressing the desire to speed, but preventing me from doing so inadvertently.
This was especially true on the portion of my commute within Washington DC, which includes many freeways (with on- and off-ramps) that have lower speed limits than one would expect due to the generally high volume of traffic.
The first time I drove home late at night in sparse traffic, the system startled me by slowing me down as I crossed DC’s 14th Street Bridge. In hindsight, I realized that the ISA device had correctly detected a reduction in the speed limit from 55 to 40 mph, a change poorly marked with a single small sign in the right shoulder lane. As far as I know, none of the 200-plus speed cameras currently deployed in the District are installed on the bridge, but the discovery was a good reminder that the ISA device not only helped me drive safely, but also helped me avoid the expense and hassle of a speeding ticket.
Another speed limit change on my commute occurs just outside the ring road. When I head towards the District, the speed limit increases from 65 mph to 55 mph. On the way home, the opposite is true. When the speed limit was lowered at this point, the ISA device responded much faster than the flow of traffic. In the other direction, the device held me to the lower limit until we reached the 65 mph sign, while other drivers drove faster in anticipation of the change. I felt like I was annoying other drivers by slowing down much earlier and accelerating much later than everyone else.
In response, I tried to change my mindset. Instead of worrying about other drivers, I kept the SUV in the middle or right lane of the highway. Faster-moving traffic started to pass me on the left, and I began to realize that driving with ISA could also lead to a less stressful commute: I wasn’t worrying about a speeding ticket, and I wasn’t fighting for position in a pointless race that might not even cut my commute time in the end.
