Why does cruise control on vehicles have a lower limit?

My current truck (Toyota Tundra) and every vehicle that I’ve owned with cruise control always has a lower limit below which the cruise function will not operate. On my truck, it’s 25 mph. If I’m in a 20 mph school zone, I’m on my own speedwise. What is the purpose for this lower limit and why is it always so high? IIRC correctly, I’ve owned vehicles where the speed could not be set below 30 mph. What gives?

Here’s a discussion I found on cartalk.com, though it doesn’t provide a definitive answer. The suggested answers involve safety issues (it’s safer to control the speed yourself in stop-and-go traffic or when driving through a school zone) and technical issues.

In the old days, the cruise control was vacuum operated and it just didn’t work at lower speeds.

Newer ones are computer controlled but they still don’t work at lower speeds. I’ve heard a couple of different reasons for this. One is that it’s a safety issue. If you are cruising through a school zone they want you to be in active control of the car’s throttle, for example. The second reason I’ve heard is that these systems don’t work so well in lower gears where the transmission ends up shifting a lot. The third reason I’ve heard is that they wanted it to work the same as the older systems so that people wouldn’t get confused by the differences.

The safety issue I can see. If a kid runs out in front of you in a school zone it is better to have your foot on the accelerator instead of flat on the floor so that you can more quickly and accurately get to the brake.

The second two reasons seem a bit iffy to me. I can’t see any reason why a computer controlled algorithm wouldn’t be able to handle the transmission shifting. It might require a bit more computer programming but it should be fairly easily to do.

I’m not sure I really buy the vacuum explanation. It’s not like the cruise control is going to be accelerating at WOT or anything. There should still be plenty of vacuum to go around at low speeds. Also I’ve had a few stick shift cars with vacuum-operated cruise control and they’ve seemed perfectly capable of trying to lug their way up a hill with the throttle completely open.

It might also be a testing cost vs. customer demand tradeoff. Why pay the money to test a product in a range where no customers would pay for the feature in that range?

With vehicle now being fitted with systems that will automatically stop the car if a kid runs out in front of you and keep you in a traffic lane, it would seem logical to be able to set the CC at 20mph or even less.

I can’t think of a reason or a safe situation to use cruise control other than on limited-access highways, where a reasonable underspeed limit is 50. It’s a feature that CC’s go much lower than that.

In fact, WOT is where the lowest manifold vacuum is observed (in a gasoline spark-ignited engine). My dad used to get pissed off on summer vacations when we’d be grinding up a long mountain grade with a pop-up camper behind us: at some point the vacuum reservoir would run out of vacuum (a small leak somewhere), and the vacuum-controlled HVAC dampers would relax and start dumping nice cool air through the windshield defrost vents instead of the dashboard vents.

IIRC it also has an upper limit. 90mph I think it was. Not that I ever drove that fast on that very long, very straight, Highway 5 between Bakersfield and Tracy.

So you were the turtle clogging that road. :smiley:

Used to drive a 20-mile stretch of 5 a ways north of that, in the early evening with other Friday-night homeward-bound commuters and weekenders. Cruising in largish wolfpacks at 100+ was not an unusual thing.

Right, that’s kind of what I was getting at with mentioning my stick-equipped cars. I know at least with my old Honda if you were going up a hill and didn’t downshift, the cruise control would happily hold the throttle wide open until the car slowed to a stop and stalled. It still had enough vacuum to operate then, so I’m sure with the throttle mostly closed holding 15 MPH it wouldn’t have had any issues.