Vehicle size and weight

Overview

A bigger, heavier vehicle provides better crash protection than a smaller, lighter one, assuming no other differences. The longer distance from the front of vehicle to the occupant compartment in larger vehicles offers better protection in frontal crashes. Heavier vehicles also tend to continue moving forward in crashes with lighter vehicles and other obstacles, so the people inside them are subject to less force.

Large vehicles aren't as big a threat to people in small vehicles as they used to be. A lighter vehicle will always be at a disadvantage in a collision with a heavier vehicle. But in recent years automakers have reduced the threat SUVs and pickups pose by more closely aligning their energy-absorbing structures with those of cars.

Fuel economy can be improved without sacrificing safety. Various technologies can raise fuel efficiency without reducing the vehicle’s weight. Manufacturers can also improve fleetwide fuel economy by taking a small amount of weight off their heaviest vehicles without significant safety tradeoffs.

How size and weight affect safety

Both size and weight affect the forces people inside a vehicle experience during a crash. The magnitude of those forces is directly related to the risk of injury.

In larger-sized vehicles, the longer distance from the front of the vehicle to the occupant compartment provides more protection in frontal crashes, which account for more than half of passenger vehicle occupant deaths. The longer that distance, the more the frame of the vehicle can be crushed before it crushes the people inside.

Weight is important when two vehicles collide. The bigger vehicle will push the lighter one backward during the impact. That puts less force on the people inside the heavier vehicle and more on the people in the lighter vehicle.

IIHS demonstrated the role of size and weight in a series of crash tests in 2019, pairing a midsize SUV and small car made by Kia and a large car and minicar made by Toyota in collisions with each other. Both of the smaller vehicles, the 2018 Kia Forte and 2018 Toyota Yaris iA, had good ratings in the five IIHS tests relevant to driver protection, but they performed poorly in collisions with the larger vehicles.

Improvements in crash protection have made vehicles of all sizes safer, but bigger vehicles are still safer than smaller ones even with those improvements. As the chart below illustrates, crash deaths decline as vehicle size increases.  A similar chart using weight instead of size would look almost the same.

Vehicle compatibility

A lighter vehicle will always be at a disadvantage in a collision with a heavier vehicle. But in the past, SUVs and pickups posed an added danger to people in cars because their bumpers and other energy-absorbing structures were higher than those of the smaller vehicles. That meant the taller, larger vehicles would effectively climb up the hood of small cars in crashes, increasing the risk of injury and death for the people in the car.

An analysis of crashes in 2000-01 involving 1997-99 model vehicles showed that SUVs and pickups were much more likely than cars or minivans of the same weight to be involved in crashes that killed occupants of other cars or minivans (O'Neill & Kyrychenko, 2004).

In recent years, however, automakers have made design changes to align the energy-absorbing structures of newer SUVs and pickups with those of cars. In 2013-16, car occupants were only 28 percent more likely to die in collisions with SUVs than with cars, compared with 59 percent in 2009-12. Though pickups are also less of a threat than they used to be, in 2013-16 they were still 2½ times as likely to be involved in a crash that was fatal for a car or minivan driver than other cars and minivans were. (Monfort & Nolan, 2019).

Balancing safety with fuel economy

Just as larger vehicles are safer than smaller ones, all other things being equal, lighter vehicles offer better fuel economy. But automakers can use technology to improve fuel efficiency without the safety tradeoff that comes with reducing vehicle weight. Electric vehicles, hybrids, auto stop/start engines and more efficient internal combustion engines are all solutions that don’t sacrifice safety, and in the case of electric and hybrid vehicles, the extra weight from their batteries can provide a safety boost.

Another strategy that doesn’t conflict with safety goals is to focus on the heaviest vehicles when cutting weight from the fleet. Government researchers have found that reducing the weights of the heaviest vehicles but not the smaller ones would not increase the risk of fatalities (Kahane, 2012). In contrast, an across-the-board cut of 100 pounds from every vehicle in the fleet would cause a 1.6 percent increase in the fatalities involving cars weighing less than 3,106 pounds. The calculations included not only fatalities of people inside a given vehicle, but also occupants of other vehicles and pedestrians.

The idea that cutting weight from the heaviest vehicles is better than cutting across the board led to a change in the government’s approach to fuel economy. In the past, manufacturers met government-mandated fleetwide fuel targets by selling more small cars to balance out the bigger gas guzzlers. But the current fuel economy requirements, in effect for 2017-21 models, use a sliding scale that requires smaller vehicles to meet more stringent targets, so selling more small cars doesn’t make it easier to hit the mark.