How Does Cruise Control Your Speed?

Cruise control. It’s the best feature in the world on the highway. At least it was until adaptive cruise came along. And sure, it might eventually get replaced by autonomous driving, but until then I’ll enjoy keeping my right foot on the floor over long highway drives. This is how it works.

Cruise control, also known as speed control, was actually invented in the 1700s, for use on steam engines. It was seen on cars as far back as the early 1900s. But what we see on modern cars was patented in 1955 by a blind engineer named Ralph Teetor and launched not long after.

Cruise Control Vacuum Control

The system maintains the speed you tell it to. You can set your speed up, down, or to just about whatever, although many systems won’t work at very slow (or really high, allegedly) speeds.

Older systems took the speed reading from a sensor on the drive shaft and used a mechanical system to open and close the throttle. They could tell if you were going faster or slower than your set speed and used a mechanical actuator to open and close the throttle. Instead of using the same cable as the gas pedal, there was often a second cable attached to the throttle plate. It was complex and expensive.

The Pinnacle of Multifunction Switch Design

Then computers came into play. The first electronic cruise control was invented in 1968. The system got fancy. More accurate too. And it was the start of being able to integrate the cruise with the powertrain computer. Once powertrain computers arrived, at least. That wouldn’t happen for a few years.

But they still worked the same way. Computer reads a speed sensor signal, knows if it wants to go slower or faster. Then it uses engine vacuum to control a valve that will pull or push on the cable that controls the throttle. If you look under the hood of a car from the mid-1970s until the early 2000s, you’ll see the two cables on the throttle. One for the pedal, one for the cruise actuator. Or, like me, maybe you scared your non-car-savvy friends by pretending to freak out about the pedal moving by itself. Another benefit of the console-free front bench.

The computer calculates not just the difference in speed, but the rate of change. If the speed starts to drop quickly, it knows it needs to mat the gas. But if it changes slowly, it will take a bit longer to accelerate. That’s more comfortable on slight rises and safer on sudden hills.

Modern adaptive systems drop the vacuum control for a motor mounted directly to the throttle. It’s one of the benefits of throttle by wire that most new cars have.

They also add lots of tech. Most systems use a radar emitter in the grille. Instead of detecting approaching enemy fighters, they detect cars in front of you by bouncing radio waves off of the car in front and measuring how long they take to return. It works up to about 500 feet away, depending on the model. They can tell if the car is in your lane, how far away you are, and how quickly you are closing. Get too close and the car will brake using a module that can control brake force. The car will let you pick a following distance that varies based on actual speed and closing speed. No need to tailgate, after all.

Drive down a hill, and the system will again brake. Some will downshift, using engine braking to slow you instead of the brakes.

So that’s how cruise control works. Like most new tech, it’s all computers. But before computers, it was still surprisingly effective. Super Cruise adds the ability to steer the car too, but we’ll save that for another time.