Two-wheel drive (2WD) describes vehicles in which, in theory, two wheels receive power from the engine at the same time. Usually the two wheels are on the same axle in the drivetrain. Most vehicles with two-wheel drive are mainly for use on roads and highways. Two-wheel drive cars and trucks are either front- (FWD) or Rear-wheel drive (RWD). This means either the front or the rear axle is the "drive axle" that moves the vehicle forward. When the vehicle begins to accelerate, some of the weight of the vehicle is shifted to the rear giving a rear-wheel drive car or better balance and traction. Two-wheel drive systems are simple and rugged. This is why rear-wheel drive is so popular in police cars and other service vehicles.