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This is more of an educational question, than a problem.

On most cars I have seen, the cruise control activation is in two steps, you usually have a separate switch to turn it on, and then you have the "Set" button to activate it.

Why is this that way? What is the purpose of being able to deactivate it in 2 ways as well - using the master switch/button, and a separate "cancel" one next to the set/restore buttons?

Also, on most vehicles if you power down the car, on next start the cruise control is deactivated, and you need to activate it again using 2 controls.

Is it bad for the engine/transmission if the cruise control is always on? I ask this, because thats how it is on most cars, but on my Honda Odyssey it does not turn of on engine restart. Its on when I start it again, if I left it on when I stopped the car.

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2 Answers 2

The purpose of being able to deactivate your cruise control in two different ways (note: there are at least 3 including brake/clutch pedal switches/neutral safety switch) is that the cruise control system has two distinct 'deactivation states':

1) System off and disabled, like the off switch on your computer. No accidental press of the set/resume buttons will interfere with your driving.

2) System off and ready, like the sleep mode of your computer. It's not currently active, but it's ready to accept commands.

The two off states are best modeled by separate buttons/switches, it would be confusing to people to have a 'two pushes for off, one push for cancel' single button.

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So, it does not cause harm to the car if it's on all the time, right? –  Sunny Apr 17 '12 at 19:16
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None whatsoever. There are two actuation methods; vacuum and electric, neither is engaged unless you're actually using it. In both of these cases, the on/off cruise control switch isn't making any current difference (if at all) and there aren't any wear components in use (like the vacuum diaphragm) when it's in the off and ready state. –  Ehryk Apr 17 '12 at 20:12
    
In fact, if you've driven a mid-90s GM/Chevy vehicle, their cruise control is on a single three position switch, OFF.ON.RESUME/ACCELERATE where ON and OFF locked, and RESUME/ACCELERATE was momentary and returned to ON. In these vehicles, the system was off and ready state every time you turned on the vehicle until you switched it off, in many cases for the life of the vehicle. Picture –  Ehryk Apr 17 '12 at 20:17
    
The SET button is separate and at the end of the stalk. –  Ehryk Apr 17 '12 at 20:18

tl;dr: Safety.

Why is this that way? What is the purpose of being able to deactivate it in 2 ways as well - using the master switch/button, and a separate "cancel" one next to the set/restore buttons?

You're asking a control theory question that's easier to answer in pictures than in words. However, SE isn't great for diagramming so I'll try to illustrate the state transitions in outline form.

  1. Initial state (e.g., immediately after turning on the car): Click the "enable cruise control" master switch to go to enabled state.
  2. Cruise control system enabled but not actively controlling speed: Click the "set" button to go to active state. Click the master switch to return to initial state.
  3. Cruise control system actively controlling speed: Click the "cancel" button to go to ready state. Click the master switch to return to initial state. Step on brake or clutch to go to ready state.
  4. Cruise control ready (target speed set but not controlling): Click the "reset" button to return to active state (NOTE: acceleration will ensue). Click the master switch to return to initial state.

The actual state diagram isn't much more complicated than that. That outline makes it pretty clear, though, that the system is designed to keep the cruise control system from affecting your speed until you tell it to at least two times. It's also strongly biased to release control if the driver seems to be taking control.

The safety aspects come into play in that ready state: that's the point at which the cruise control is prepared to (potentially dramatically) increase your speed. You only want this to happen when you say so and you certainly want to be able to say "whoa!" immediately if that speed is too high for current conditions.

Regarding systems that can default to "on": I know that some cars have a mechanical cruise switch that can stay on "on" after power down. Personally, I regard that as a dangerous feature. I like the default setting that cruise control will ignore all of my inputs until I specifically wake it up.

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So, it does not cause harm to the car if it's on all the time, right? –  Sunny Apr 17 '12 at 19:16
    
Correct: nothing about the cruise control damages the car. –  Bob Cross Apr 17 '12 at 20:58
    
Well cruise control can damage your car if there's a stopped vehicle (or other object) right in front of your car... :-) –  R.. Apr 22 '12 at 20:43
    
@R.., no, that's operator error. That's as if you released the parking brake on a car in neutral and blamed the brake when the car rolled into a ditch. –  Bob Cross Apr 22 '12 at 22:37

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