Unlimited
December 31, 2025
Alex KiersteinAn Arizona rep hypes unrestricted speed limits, claiming it’s safer than once feared. There are many counterarguments.
This month, an Arizona representative introduced a bill (link to PDF file) that has been garnering a lot of media attention, even though it’s just in the earliest stages of the legislative process. It would, under certain circumstances and in certain areas, eliminate speed limits on certain stretches of Arizona interstates. The rationale is that it’s actually safer, in contradiction to much of the messaging we’ve received over the years. Given the subject matter, the attention is understandable–though it should be noted that legislators can introduce whatever bill they want and often do it just to get their name in the paper.
Rep. Nick Kupper’s bill, HB 2059, has a nickname (the RAPID act, or Reasonable and Prudent Interstate Driving Act) and a press release that puts the move in both the context of rural versus urban sensibilities and rsays it reflects the “reality” of freeway driving in less populated areas. It is heavily reliant on Montana’s experience using the 85th percentile model of setting limits, and the studies it conducted or referenced showing that speed differential (i.e. reducing the difference in speed between vehicles) is more consequential for safety than absolute speed.
It also heavily references Montana’s experience with no statewide speed limit. Before the institution of a national 55 MPH limit and then after its repeal in 1995, Montana had no daytime limit on highways, the requirement was that drivers should go “at a rate of speed no greater than is reasonable and prudent.”
That language is enshrined in Arizona’s motor vehicle statues, saying regardless of the posted speed limit, a driver’s speed should take into account the conditions. That’s prudent; don’t go 80 mph over black ice. But remember, in Montana, there was no speed limit. So “reasonable and prudent” required the judgement of the driver and the discretion of law enforcement.
Montana’s unrestricted speed experiment ended in its state supreme court with an improbable plaintiff and an even more improbable outcome. It’s a story that deserves to be told in its own right—and thankfully, the Cowboy State Daily has it all right here.
The reasonable and prudent standard gives a little wiggle room for enforcement when the ultimate speed is deemed unreasonable. Essentially, it’s an argument that if everyone is going to drive at 85 mph plus or minus 5 mph, let’s ensure that traffic flows close to that speed, so there aren’t accidents involving a great disparity (such as someone trundling along at the posted 65 mph speed limit and encountering someone going 85).
There is not universal consensus that unrestricted rural interstate driving is safer. A AAA study in 2024 examined whether there is a significant adverse “spillover” effect in communities along routes with higher speed limits. So it’s not as simple as just reducing differential speed issues. There is also evidence that increased speed limits lead to increased crashes. There are also studies that contradict that. The IIHS is very clear that speed is a major factor in traffic safety in general.
There are three undeniable factors concerning speed limits that seem to be woefully unaddressed in most coverage of both the Arizona bill, which was just introduced and is not close to becoming law, or Montana’s high current speed limits and unrestricted past ones.
The first is that raising speed limits has a minimal effect on travel times. The second is that human reaction times and the principles of physics are basically immutable. Higher speeds can mean significantly less time to react to changing conditions, traffic, etc. And, in the event of a crash, higher speeds usually mean greater risk of injury to vehicle occupants.
Though, the study just cited provides a good reminder that studies are often designed to prove a point, not necessarily to uncover some universal truth. It states that higher speeds on rural highways correlates to more severe outcomes (i.e. a greater incidence of fatalities), but that the data on whether crash frequency increases isn’t clear.
And what about efficiency? At high speeds, to save a few minutes, drivers will burn more gas (or use more electrons). The cost of that can be quantified. And there’s the psychology of traffic speed. The 55 mph national speed limit was generally reviled. Drivers appear to have unrealistic ideas about the relationship between speed and safety.
I don’t have a solution to suggest, but I will point out that it seems simplistic to create unrestricted speed zones without considering a whole host of other factors. I haven’t even gotten into the engineering recommendations on speed limits based on the infrastructure and setting, for example. Basing the majority of the rationale on Montana’s experience, with a heavy reliance on the idea that speed-differentials will be reduced, which will necessarily reduce crash frequency a little flimsy.
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