A car has a pedal called "the accelerator." Indeed, you "step on the gas" to go faster. However, a car is not quite an isolated system, especially at high speeds where it becomes hard to push the air out of the way fast enough; then "pedal to the metal" just means to go fast, rather than to accelerate constantly. Much more consistently we could call the brake pedal "the accelerator" (or perhaps the "decelerator") since it always causes the velocity to change.
A familiar acceleration-detector is a very full cup of coffee. Moving at constant velocity is perfectly safe -- after all, the airline steward pours coffee at 500 mph, without thinking about it! -- but any deviation from constant speed or constant direction is sure to cause trouble.