The change in a vehicle's speed during a crash.

"Speed" here means velocity, so delta-v measures how much the vehicle's motion changed, not just how fast it was going before impact. "Change" matters because a car traveling 50 mph that glances off another vehicle may have a much smaller delta-v than a car hit hard at a lower posted speed. And "during a crash" matters because delta-v is a reconstruction number, usually estimated from vehicle damage, event data recorders, crush measurements, and scene evidence - not from guesswork, and not from a bumper photo on a phone.

People often hear that a "low delta-v" crash cannot cause real injuries. That is bad advice. Delta-v can help engineers discuss crash forces, but it does not settle whether a person was hurt. Body position, seatback angle, head turn, prior injuries, age, and whether the hit was expected can all change what happens to a human body inside the vehicle. A modest change in speed can still leave someone with serious soft-tissue injuries, a concussion, or worse.

In an injury claim, delta-v usually shows up in fights over causation and damages. Defense experts may use it to argue the crash was too minor to matter. That argument is not automatic truth. In Maryland, there is no statute setting a delta-v threshold for bringing a claim. Police agencies such as the Maryland State Police may investigate the wreck, but delta-v by itself does not decide liability, credibility, or whether medical treatment was necessary.