Automatic Cruise Control
Subaru of America, Inc.

Cruise control is one of those features that most of us cannot live without, but have you ever wondered how cruise control works? How is it able to keep the speed you have set, even when climbing up a hill?

It isn’t black magic, but a system of various mechanical and electrical parts that work together. We’ll be taking a closer look at the standard cruise control system fitted to most vehicles. Plus, we’ll dive into the latest advances in cruise control technology, such as adaptive cruise control.

The Ins and Outs of Basic Cruise Control

Cruise control begins with a set of buttons on the steering wheel that allows the driver to turn the system on and off, set the desired speed, and either increase or reduce speed.

The driver sets the speed by pushing the accelerator pedal until the desired speed is reached and hits the ‘Set’ button. Depending on the age of the vehicle, there are a couple of different ways the cruise control system keeps the set speed. Older models use a cable that is hooked up to the throttle, which controls the speed of a vehicle, and an actuator, which moves the cable back and forth. The actuator itself is controlled by the vehicle’s ECU (engine control unit), which monitors the speed you have programmed and the speed of the vehicle from wheel sensors.

Say you set the cruise control to 70 mph. The ECU monitors the speed of the vehicle and makes sure the vehicle gets up to the set speed and stays there. If all of a sudden the vehicle starts climbing a hill and the speed decreases, the ECU will see this and have the actuator pull the cable to open up the throttle and get the vehicle back up to speed. Once up the hill, the ECU tells the actuator to let out the cable and reduce the throttle. This works the same when the ECU detects the vehicle going faster than the set speed (if you're going downhill, for instance). The ECU will alert the actuator to let out the cable to reduce the throttle. In modern vehicles, the throttle is controlled is controlled by electric motors instead of a cable, but the basic process remains the same.

Adaptive Cruise Control

For a number of years, a big problem with cruise control was that it didn't work well in traffic. You had to constantly turn it off because a car cut you off or traffic slowed down. Plus, there was always the issue of causing an accident if you became distracted while it was on.

However, automakers had a solution in the form of adaptive cruise control, or automatic cruise control. Adaptive cruise control uses a sensor – either radar or laser – to monitor the road and detect vehicles. The driver sets a speed and a distance they want between their vehicle and the vehicle in front of them. If the system detects a vehicle within that distance, it will notify the ECU to slow the vehicle down to keep the set distance between the two vehicles. The ECU will keep the vehicle at a lower speed until the vehicle in front speeds up or changes lanes.

In 1999, Mercedes-Benz was the first to introduce an adaptive cruise control system. Other auto manufacturers would soon follow. At the beginning, there were two systems available: radar and laser. Laser systems were used by many automakers, as they were slightly cheaper than radar. But laser systems had a number of issues, such as not being able to reliably detect a vehicle in rain or snow. You also had to make sure the sensors were clean. After a few years, radar systems got down to the same price as laser systems, and many automakers began to switch.

Subaru EyeSight

What’s in Store for the Future?

Cruise control is going to be playing a key role for autonomous vehicles. Adaptive cruise control systems will be used to monitor vehicles in front to keep a safe distance. Even though we are still a ways off from autonomous vehicles, various technologies have trickled down to current models.

One example is Subaru's EyeSight system, which uses stereoscopic cameras instead of radar for adaptive cruise control. Camera systems can offer a longer range (about 87 yards in front of the vehicle) than radar systems. Newer adaptive cruise control systems can also bring the car to a full stop if necessary. Subaru has recently updated their system with new color cameras. This helps the system better recognize certain objects, such as a vehicle’s stop lights.

Other manufacturers, such as BMW, have implemented automatic stop-and-go driving into their systems. Say you’re driving on the freeway and you come to a traffic jam where no one is moving. The system is able to recognize this and bring the vehicle to a full stop. Once traffic starts moving, the system will automatically accelerate back to the speed you set.