The following is contributed as added information, and not an attempt to contradict anything mentioned by previous posters.
There have been, at times, some disagreements about the various contributing factors in Dale Earnhardt’s fatal clash.
As I remember, and as generally supported by looking over a few relevant articles from the time (found by searching on “dale Earnhardt cause of death”), the following bits are generally accepted:
–The autopsy reported the cause of death as blunt-force trauma to the head and neck. Earnhardt had a sizable skull fracture around the base of his skull that also extended to the sides of his skull. He had internal bleeding at the base of his brain, and brain swelling. He also had 8 broken ribs, a broken breastbone, and a broken left ankle. He also had various body contusions, partially collapsed lungs, and blood in his ears and chest. Emergency crews reported no pulse or signs of life when they arrived at the scene.
–The car steering wheel, with a rigid metal core, was bent on the right side from the impact of Earnhardt’s face/torso, and also dented near the bottom from the impact of the safety harness clasps.
–Earnhardt, as usual, was wearing an open-faced helmet, with no chin or face protection.
–Earnhardt used a driving position that required his seat to be mounted further back from the steering wheel than any other NASCAR driver at that time.
–Earnhardt had a reputation for being particular about how his safety harness (seat belts) were installed and fitted, and for requiring them to be installed in a custom way. However, there is no definitive evidence of how loose/tight his harness was before the incident, or that that variable had anything to do with the severity of his injuries.
–The safety harness was NOT installed the way the manufacturer specified it be installed.
–One of the first responders to the accident says that the belts were intact when he released the catches. NASCAR says one of the belts failed in the webbing (not at a junction with hardware) at some time during the accident. The harness manufacturer says if it failed during the accident, it is because it was improperly installed.
–As this was clearly a racing accident, the car was not handled/treated as a crime scene on a tv show, and thus a lot of questions never were and never will be resolved.
----NASCAR’s initial investigators said that Earnhardt had a good chance of surviving the crash had the belt not broken. One massive lawsuit (brought by the belt manufacturer and settled out of court) later, NASCAR quit saying that.
–As a result of a different lawsuit, an independent medical expert from Duke University, Dr. Barry Meyers, was engaged to study the death. He concluded the death was directly caused by Earnhardt’s head and neck snapping forward in and of itself, and was independent of the broken (or not) belt.
As mentioned by others, a lot of safety improvements were researched and implemented following this particular incident, mainly due to the overwhelming notoriety and popularity of Earnhardt and the resulting public outcry, especially after two NASCAR driver fatalities (Kenny Irwin and Adam Petty) the previous year caused by similar basal skull fractures after cars hit the wall.