Seatbelts and airbags in modern cars are basically doing what a helmet tries to do which is protect your head from hitting something hard (like the windshield o

Seatbelts and airbags in modern cars are basically doing what a helmet tries to do which is protect your head from hitting something hard (like the windshield or steering wheel etc. In a modern car driving along a normal road, I don't think a helmet would add much additional protection. Speaking from experience treating traumatic brain injuries, the brain injuries that are common in a car (as long as you are wearing a seatbelt and don't get the) is something called diffuse axonal injury (DAI) also know and traumatic axonal injury (TAI) which is diffuse brain injury caused by the (de)acceleration/rotational acceleration. A helmet would not be much help against those types of injuries.