In reconstructing events in a fatal crash on Route 3 in Anne Arundel County, investigators obtained key evidence from a source many people probably don't know even exists: an air bag control module.
The obscure part in the 2008 Chevrolet Cobalt driven by Elizabeth Haley Meyers showed the car came to a full stop before crossing the busy highway in front of a motorcycle that struck the car. That evidence refuted the statement of an eyewitness who told police he saw her texting without slowing down before pulling onto the highway from a shopping center.
With a key witness' statement in doubt, prosecutors had a much weaker case, and on Monday agreed to allow Meyers to plead guilty to one count of negligent driving. Manslaughter charges were dropped.
Prosecutors could only prove that in last spring's crash Meyers "pulled out without making sure the coast was clear," said Anne Colt Leitess, Anne Arundel County state's attorney.
Investigators have been able to use data from air bags for about 10 years, said Lt. David Ennis, commander of traffic safety for the Anne Arundel County Police Department.
Various vehicle makes and models offer different data and not all manufacturers give police the tools to access the data, Ennis said.
A vehicle's air bag control module constantly reviews data to determine whether any of the air bags should be deployed. When there is a crash and an air bag is activated, the most recent data is saved in the module and investigators can copy or "image" the data for use in their crash reconstruction, Ennis said.
Most air bag control modules capture about five or six seconds' worth of data such as engine revolutions per minute, speed, how much throttle was applied, whether the brake was engaged, whether there was a change in velocity and if a seat belt was fastened.
The amount of data available is constantly expanding. Some air bag control modules now record downward and lateral forces, Ennis said. "They're getting more sophisticated every day," he said.
But Ennis cautioned that air bag control module data alone can't tell the full story of a crash. He said the best use of the data is to verify calculations made using physical evidence such as skid marks, distance traveled and the weight of a vehicle.
"It's not the end-all, be-all," he said. "It still has to be used with mathematical calculations and physics calculations."