Some OEMs offer such a weight sensor technology as part of a more complicated occupant sensing system, and use the information to adjust the airbag and seatbelt pretensioner deployment power based on occupant weight, size, position, and whether they are buckled-up, e.g. softer for smaller lighter occupants or occupants unbelted, (testing acording to US regulations), more aggressive for larger heavier occupants. There is also a sensor that detects the tnesion in a seatbelt to make similar changes deployment characteristic changes, this is called a Belt Tension Sensor. When you go to purchase a new car, ask the dealer what systems are available for the cars he sells. If he doesn't know, check out the vehicle manufacturers website for the car you are after for the safety system specification.