Seat belts are made from polyester webbing that includes 300 warp strands and one weft strand. The belts, which are almost 2 inches wide, are designed to restrain more than 6,000 pounds without breaking.
Seat belts are wound around retractors that lock when vehicles suddenly decrease speed or when they accelerate above a preset limit. The retractor also locks the belt when it is pulled out quickly, as in the case of a collision. Modern vehicles usually also include a pretensioner device, which reduces the seat belt's slack to firmly hold a passenger during a collision. A pretensioner device also limits the force a seat belt applies on a passenger to prevent damage to the passenger's body.