I guess it depends on the routes you take. Crossing the Nullarbor, covering long distances on the Atlantic Coast of Patagonia south of Valdes, or travelling on any other long road with little traffic, having cruise control is far more comfortable than having to maintain your speed "manually" for hours (or days).
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Being a former prototype driver, where you spent 10 hour shifts behind the wheel, Cruise control was a welcome option. It is so easy to either slow down or creep above your dictated speed. Especially where slight descending inclines would cause a higher than wanted speeds over posted restrictions, Aka......75 mph, but with a very gradual grade, you could be doing 83 mph without realizing it. Some might think it is easy to maintain a set speed by your foot, but doing so mile over mile, hour after hour is not. You find your eyes to often looking below the dash line at your speedometer, instead of out the windshield. This is out west, with long stretches of fairly straight roads occur. So it becomes a safety issue, and not just a legal one.. Most rural state or interstate roads are 70 or 80 mph in NV, AZ, Also realize this, on a 10 hour shift, on a set route, crossing county and state borders, you were expected to maintain an average speed that allowed you to complete, and at shift end, hand off the vehicle to the next shift as these were prototype vehicles doing durability testing and they were wired with a complete sensor package array. Also speeding was severely discouraged, so for employment reasons/issues , that cruise control button made life much easier when it was not disabled. Yah, the engineers did do that depending on the car and test parameters.
Concerning adaptive cruise, we were testing it 3 years before it started becoming available. Think Mercedes and Porsche. For the open highway it was nice,. For freeway use....uh not as much, as depending on the space you set, some idiot would squeeze in front of you, causing your car to suddenly brake, and risk being rear-ended by the car behind not paying attention. A short coming on adaptive cruise in city use, is say your following someone, your cruise is set at 30 feet behind the car in front of you, your doing 45mph and they make a sharp right turn. That car clearing your front cruises sensors, will automatically cause yor car to speed up to your set speed. If that car has done it slowly, you risk 'cliping' it because of it being slow to completly clear your lane, and your car automatically speeding up to your set speed. We had a few close calls as they worked the bugs out, from a engineering aspect. I never trusted the system completely . It was one of those things you could get to used to, and be caught flatfooted given certain conditions, As some on this thread stated, I prefer the older cruise control (on/off) vs adaptive, given a choice.