Conventional wisdom says, all other things being equal, surrounding yourself with the most metal possible will protect you the best in a crash. And that has been borne out by some studies, and assuming bigger is better in a crash is still a reasonable rule of thumb up to a limit. Even the Insurance Institute for Highway Safety (IIHS) says so. 

There are many caveats. Massive vehicles are much, much more dangerous for pedestrians, bicyclists and any other living or smaller thing on the road. And the IIHS also acknowledges that making these large, heavy vehicles lighter wouldn’t affect safety as much as you think. It’s not a 1:1 relationship between vehicle mass and vehicle safety, in other words. Lastly, when a large vehicle strikes a small one, the occupants of the smaller vehicle are disproportionately killed. For example, since 2005, a pickup truck striking a regular car is 159 to 212 percent more likely to result in the death of an occupant of the car, compared to car-to-car crashes, according to IIHS.

More from MotorTrend:The best used SUVs under $15,000: Finding utility on a budget

A new study by IIHS puts a bit of detail on the numbers and assumptions involved. Basically, as the safety systems in all vehicles have improved — think airbags, automatic emergency braking and the way bumpers are designed — the role of vehicle weight has declined. In other words, occupying a heavier vehicle doesn’t offer as much of a benefit as it once did because of all the systems preventing accidents, or mitigating the effects, that all modern vehicles are increasingly equipped with.

The study used Fatality Analysis Reporting System (FARS) data from 2011 to 2022, focusing on vehicle crashes in which the vehicles were between 1 and 4 years old and there was at least one resulting driver fatality. (In a somewhat sobering process, the total of 440,604 traffic fatalities in that timeframe was winnowed down to 9,674 deaths by this process.) 

Need a break? Play the USA TODAY Daily Crossword Puzzle.

The result is really interesting. There’s a divergence at a specific weight point — about 4,000 pounds. Below that threshold, IIHS found that heavier vehicles confer safety benefits to the driver that increase by weight, but the fatality rate in the other vehicle increases at a comparatively small amount. That is to say, a heavier vehicle is safer for the driver, and only a little bit more dangerous to other vehicles, up to about 4,000 pounds. The IIHS example here is illustrative: “a lighter car gaining 500 additional pounds was associated with a 17-point reduction in driver death rate and only a single point increase in car partner driver death rate.”

In case you missed it:The best and most disappointing cars, trucks and SUVs of 2024

Above that two-ton threshold, things go the other direction. Drivers of these vehicles don’t see an additional safety benefit for any additional weight, and the fatality rate of the vehicle it impacts increases significantly. 

The conclusion? There’s a clear upper limit to vehicle weight when it comes to maximizing a driver’s safety. IIHS concludes that reducing the weight of the largest vehicles could actually benefit driver safety, and of course improve the fatality rates of the vehicles they might hit. Reducing vehicle mass also has a number of knock-on benefits, like improving fuel economy and perhaps payload or cargo capacity, depending on other factors. But the bottom line is that reducing the weight of the heaviest vehicles on the road will almost certainly save lives. 

Photos by MotorTrend