Have you ever been on a long car trip? If you know how fast you're traveling and how far you have to go, you can predict how long it will take you to get there. That's because your speed, the distance you travel, and the time it takes to get there are part of a linear relationship.
Cruising Down the Highway
If you drive at a constant speed, you can accurately calculate how far you've traveled. Things can get complicated if your car changes speed a lot. If so, you'd need to figure out your average speed to find out how far you've traveled. Changing speeds frequently doesn't just make it harder to estimate travel times—it also wastes fuel and can lead to accidents.
To make long drives safer and more efficient, many cars have cruise control systems that can keep the car at a constant speed. Once activated, a computer measures the car's speed and automatically controls the engine. It helps the car travel at a constant rate, even on hills.
Cruise control works well on the open highway, but you can't use normal cruise control in stop-and-go traffic. Modern systems use radar. These new cruise controls not only measure speed but also the distance to the car in front of yours. Thus, if the car ahead decelerates, the system can slow your car down as well. New advances in cruise control may help make self-driving cars possible.
See for yourself: http://www.youtube.com/watch?v=30bF_5HE0QI
Mariah is on a road trip, driving at 60 miles per hour. She's activated cruise control, and it's 200 miles to her next destination. How long will it take her to reach the halfway point of her trip?