Your car or mine?
The Takeaway
If both cars can be inspected/downloaded, they should be.
If only one car can be inspected/downloaded, it should be.
Damage and electronic data from each car reveals important information about both cars and the crash as a whole.
One warm Summer afternoon, several years back, I found myself outside of Minneapolis investigating a crash between a pickup truck and an SUV. The plan was to inspect both of the vehicles, download their event data recorders, and then visit the crash site before heading back to the airport the next day. My first stop was a tow yard in an industrial part of town where the SUV was supposed to be. Long story short, it wasn’t there anymore and the inspection wasn’t going to happen. I loaded back into the rental car and set off for inspection number two - the pickup.
The pickup was well off the beaten path of a rural town, where it was being stored at the owner’s home. The news of the missing SUV had traveled fast and reached the owner of the pickup during my drive to his house. He had agreed to allow me to inspect his vehicle and download the crash data stored in its event data recorder under the condition that the other driver’s SUV would get the same treatment. That was fine by me, preferred even. It was in everyone’s best interest to collect all of the evidence.
The mood started off pretty tense when I stepped out of the rental car. The pickup owner wasted no time making his problem with my presence clear - he felt as though he was the only one laying his cards on the table as the only one having their event data recorder downloaded. The skepticism was understandable but crash investigations are nothing like a game of poker. The cards he was holding - the pickup’s crash data - held answers about the incident as a whole, not just his part in it. Both vehicles contribute something to the crash, both vehicles leave their mark on the other, and either one of the vehicles can answer questions about the other and the crash as a whole.
The Scale Doesn’t Lie
Step on the bathroom scale and what do you see? Forget about what the number is for a moment and focus on what the number means and how the scale determined it. The number, of course, represents your weight. By definition, weight is just a measure of the force between our body and the ground we’re standing on (under the acceleration of earth’s gravity, of course). So what the scale is really measuring is how much force your body is pushing down on it. Think of it this way, if you were to press your hand on the scale and push down increasingly harder (i.e. with more force), the number on the scale would go up because the scale is measuring the force being applied to it.
Force is not a one-way street. When you stand on the scale, putting the full force of your body onto it, the scale is pushing back on your feet with the same amount of force. This can be difficult to picture but it’s an essential law of physics (Newton’s Third Law, actually). The scale is holding you up when you stand on it. You might say it’s supporting your weight. How much force does it take to hold a 40lb dumbbell in the air? It’s obviously 40lbs, right? Any less than that and the dumbbell is staying on the ground. You’ll undoubtedly feel every bit of the 40lbs pushing back down on your arm just as the weight of your body was pushing down on the scale. From scales, to dumbbells, to car crashes - the concept is the same. Forces come in pairs that are equal in magnitude.
Back to Cars
In a crash, both vehicles experience the same amount of force. It doesn’t matter how the crash occurred or which parts of the cars were involved, the force of the crash is the same for both vehicles. Even if one car seemed to crumple like a tin can while the other looked barely damaged - they both experienced the same amount of force. The extent of the damage to each car is just a reflection of how they handled the force. Like smashing a soda can with a rock - the thin metal can collapses under the same force that didn’t leave a mark on the solid rock.
When it comes to crash investigations, every mark on each of the vehicles tells us something about how they interacted with each other just as the marks left on the road tell us about what happened long after the cars were towed away. Downloading a vehicle’s event data recorder is no exception, knowing how fast just one of the cars was going solves an essential part of the equation for determining how fast the other car was going. Event data recorders also record the force of the crash and report it as a value called the Delta-V, a measure of how much the vehicle’s speed changed during the collision.
Getting a little technical: A change in speed over a particular amount of time is the definition of acceleration. Force is defined as a mass multiplied by an acceleration (thanks again, Sir Isaac Newton) so, while Delta-V and force aren’t the same, they’re tied to each other. Simply put, knowing the Delta-V experienced by one vehicle allows an accident reconstructionist to find the Delta-V experienced by the other vehicle as well as the force experienced by both vehicles.
Final Thoughts
So, your car or mine? Which on should be inspected? Ideally, the answer is both! More evidence is always better than less when you’re looking for answers. But if one car goes missing, or becomes unavailable, it is still absolutely worth pursuing an inspection and event data recorder download of the other car. Each mark on the car and each piece of electronic data recovered is a piece of the puzzle that forms a picture of the crash as a whole.