Adaptive Cruise Control, or ACC, is a driver assistance system that helps a car keep a chosen speed while also maintaining a safe distance from the vehicle ahead. It matters because many rear-end crashes happen when drivers react too late or follow too closely. ACC uses sensors and a computer controller to watch traffic in front of the car and adjust the throttle and brakes.
The goal is smoother driving, less fatigue, and a more consistent following gap.
Key Facts
- Following distance can be described by time gap: time gap = distance to lead car / follower speed.
- A common safe setting is about 2 s to 3 s of time gap in normal conditions.
- ACC measures range, which is the distance to the vehicle ahead, and range rate, which is how fast that distance is changing.
- Relative speed = lead vehicle speed - follower vehicle speed.
- If relative speed is negative, the ACC vehicle is closing in and may need to slow down.
- Stopping distance increases with speed because braking distance is approximately proportional to v^2.
Vocabulary
- Adaptive Cruise Control
- A vehicle system that automatically controls speed to maintain a selected cruising speed or following distance from a vehicle ahead.
- Radar Sensor
- A sensor that sends radio waves forward and uses their reflections to estimate the distance and relative speed of objects.
- Time Gap
- The amount of time it would take the following vehicle to reach the lead vehicle's current position at its present speed.
- Actuator
- A device that carries out a control command, such as opening the throttle or applying the brakes.
- Control Algorithm
- A set of programmed rules that uses sensor data to decide how the vehicle should speed up, slow down, or maintain speed.
Common Mistakes to Avoid
- Assuming ACC drives the car by itself is wrong because most ACC systems only control speed and following distance, while the driver must still steer and watch the road.
- Using distance only and ignoring speed is wrong because a 20 m gap is very different at 20 km/h than at 100 km/h, so time gap is usually the safer measure.
- Thinking ACC can stop instantly is wrong because tire grip, road conditions, brake limits, and reaction of the control system all affect stopping distance.
- Relying on ACC in poor visibility or bad weather without caution is wrong because heavy rain, snow, sharp curves, or dirty sensors can reduce detection accuracy.
Practice Questions
- 1 A car using ACC travels at 25 m/s and is set to a 2.0 s time gap. What following distance should the system try to maintain?
- 2 An ACC car travels at 30 m/s behind a lead car traveling at 24 m/s. What is the relative speed using relative speed = lead vehicle speed - follower vehicle speed, and is the gap increasing or decreasing?
- 3 Explain why an ACC system should use both distance and relative speed instead of using only the measured distance to the car ahead.