Cruise Controls Cut Out When You Need Them Most

When I drive, I use the cruise control to keep me from speeding. I set up the car to go exactly the posted speed limit, so I can pay attention to traffic rather than monitoring my speed. Even though it annoys all the drivers around me, it works pretty well: I haven't gotten a speeding ticket since 1983.

But there's a problem: school zones. Sometime in the hazy past when software was just beginning to take over mundane tasks from custom analog and digital circuitry, the person who was given the task of programming the cruise control thought that it should not engage until the car reached a speed of 25mph.

But school zones have a speed limit of 20mph, and this is where you need cruise control the most. As you drive through a school zone, 100% of your attention should be devoted to not running over the little kiddies and 0% of your attention should go to looking at your speedometer. To make matters worse, one time in five there's a police car posted at one end or the other with the radar running. At least with me behind the wheel, having the police car there makes the kiddies less safe, not more so.

However, I have noticed that once you get the cruise control to engage at around 25mph, you can click the decelerate button repeatedly and get it to actually hold the car at a steady 20mph. If you think ahead, you can get the car to go through a school zone safely. But beware! If you dip below 19mph, the cruise control will cut out and then you're sunk.

Why 25? why 19? These are just numbers. I've seen the code*, and they are right there as manifest constants. To the programmer who chose them, they might have sounded good at the time. But that's only because nobody raised the School Zone Question. And now we are stuck with them forever.

The first car to exhibit this behavior was my 1991 Mazda 626. My 1994 Dodge Caravan did the same thing. Ditto the 2005 Nissan Quest, the 2008 Hyundai Elantra, the 2015 Toyota Prius and the 2021 Honda Pilot. The software cannot be common between all these models, because the newest cars now tell you via a pop-up alert that they are refusing to set a speed lower than the arbitrarily chosen limit. They don't give a reason, but then that would be impossible given that the choice is arbitrary. Even so, the magic numbers have been repeated and copied ad absurdum.

I know the numbers are arbitrary, because I recently learned from a friend that the cruise control on a Tesla can be set to an absolute crawl. As long as they were redesigning the source of motive power, I guess they thought "Why not design a sensible cruise control as well?"

The gauntlet has been thrown down. The next car I buy will have a cruise control that I can set at 20mph without having to jump through hoops. If buying a Tesla is the only option, then so be it.

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*The source code for one version of an actual cruise control was used as toolchain test code while I was working for ZiLOG between 2001 and 2007.