President Obama's fuel efficiency mandate will force auto manufacturers to produce smaller, lighter vehicles, and these vehicles have been shown to be inherently less safe than larger, heavier vehicles. If thousands of paper-thin subcompacts were to replace hefty trucks and SUVs on the roadways, the number of auto fatalities is likely to skyrocket.

A study from the Insurance Institute for Highway Safety finds that larger, heavier vehicles are inherently safer than smaller, lighter vehicles. And it's all due to the laws of physics.

When a large, massive object collides with a smaller object, the larger object, by virtue of its greater mass, inflicts more impact. "The bigger, heavier vehicle will push the smaller, lighter one backward during the impact. This means less force on the occupants of the heavier vehicle and more on the people in the lighter vehicle," the report states. "Greater force means greater risk, so the people in the smaller, lighter vehicle are more likely to be injured."

For a case study, the report evaluates the head-on impact of two vehicles with varying size and weight. The larger vehicle weighs 3,600 pounds; the smaller vehicle weighs 1,800 pounds. When the vehicles collide head-on at a rate of 40 mph, the heavier vehicle pushes the lighter vehicle backwards at 13 mph. The velocity change of the lighter vehicle is 53 mph, more than twice the change of the heavier vehicle's change of 27 mph.

Passengers in smaller, lighter vehicles have been shown to be at a disadvantage in accidents. In fact, in both single and multiple vehicle accidents, driver fatality rates decline dramatically as vehicle size increases.

But vehicle safety doesn't rely solely on mass. Some experts believe good engineering and strategic design are more important to vehicle safety than size. Features such as front and side airbags, seatbelts with pre-tensioners and force-limiters, head restraints, rollover prevention mechanisms and crash avoidance features play a crucial role in enhancing vehicle safety.

The size of a car's front end plays a role in vehicle safety, too. The safety hazards of a lighter vehicle can be offset by increasing the size of the vehicle's front end, specifically the distance between the front of the vehicle and its occupant compartment. A vehicle with more energy-absorbing "crush space" takes longer to stop during impact, thereby decreasing the severity of impact and easing the force on occupants.

A vehicle's weight can be reduced without sacrificing much structural integrity by manufacturing parts with aluminum, titanium or plastic. But most carmakers shy away from these materials due to their high cost.

The bottom line? For vehicles to satisfy President Obama's fuel efficiency standards mandate, manufacturers will have to produce lighter vehicles in the coming years. And even if Americans refuse to give up their SUVs, the models they drive are likely to be significantly lighter.