Affected by the turmoil in the economy, job losses, and increasing oil prices, consumers have drastically adapted their car buying habits from the days of behemoth SUV to smaller, lighter cars. Since 2006, carmakers have added more subcompact and micro cars to their fleets, giving car buyers the lower purchase price and fuel economy they have been in search of. At least this is what most of us observed and surmised as fact. But what does this transformation mean and how momentous was the shift?

Data points readily available from RL Polk tell us what vehicles by weight consumers are purchasing and bringing into the on-road fleet. We took this detailed volume and weight data and created a weight-by-vehicle index to pattern the relative purchases over time (normalized for volume differences).

The trend toward heavier vehicles continued through 2007 and strongly into 2008, when most realized the recession was upon us and was going to wield a significant impact. Heavy vehicle purchases dropped precipitously while medium-weight purchases remained constant and lightweight purchases rose significantly.

What does this mean with regard to injuries sustained in accidents?  There are two major concerns. First, the probability of significant weight differential in accident vehicles increases as more subcompact and micro cars are added to a fleet bloated with Suburbans and Expeditions. Data from the Insurance Institute for Highway Safety tells us that more deaths occur in lighter vehicles than heavier vehicles. Using the same logic, more injuries are likely to occur when the disparity of vehicle weights is significant.

The second issue is the relative height of vehicle crash points. A bumper on a sizeable SUV, such as a Suburban, is much higher and the mass behind it much greater than the bumpers on a subcompact like a Honda Civic. Bumper-to-bumper crashes in this instance are almost impossible, particularly if the SUV or light truck driver has lifted the vehicle, which also creates the potential hazard of an SUV bumper overriding the side impact beam of many small cars. The subcompact car submarines the larger, heavier vehicle, missing the crash points that have been built in to absorb the impact, sending the larger vehicle toward the passenger compartment.

Vehicle weight and height, along with physics, foretell more injuries and higher severity until safety measures are implemented to mitigate the height differential and more of the current fleet moves from heavy to medium and light vehicles.

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