One way to compare the relative safety of vehicles is to look at driver death rates. From time to time, IIHS researchers compute the rate at which drivers of individual models are killed in crashes. Rates are given as the number of driver deaths per million registered vehicle years. (A registered vehicle year is one vehicle registered for one year.) To increase the exposure and thereby improve the accuracy of the calculations, results are included for the previous three model years if the vehicle wasn't substantially redesigned during that time. These calculations take into account only deaths of drivers, not passengers, since every vehicle that crashes has a driver, but not every vehicle has passengers.