I travel over a mountain very frequently and I use cruise and I'll set it at like 45 but it'll get up too 55 going down hill is that normal behavior? For this cruise control system?

 

Yup it happens on mine. Its a bit bad design especially since the RX has a adaptive cruise. I noticed if coasting down a steep grade. It will downshift to slow down, but it will allow the vehicle exceed the set speed what I consider significantly +7mph. It seems to apply the brakes when another vehicle is in its path to bring the speed down. Not sure why it just does not use the brakes and regulate the speed to the set speed. Thats just poor design since the system has the capability to brake.

Its disappointing because my 2010 without adaptive cruise had this same overspeed issue on steep grades. Though I was ok with it as I understand that it probably could not brake to reduce speed as a hardware limitation so it would just downshift, but with adaptive cruise and no technical limitation to apply the brakes its disappointing engineering this time around. Also to further crap on Lexus cruise control my 2005 Audi A6 no adaptive held the cruise control set speed with brakes. that was 11 years ago (2016 RX) not sure why Lexus engineers did not do this. Also with adaptive cruise the gap to car in front is to large in slow moving traffic.

JOKE: Thank goodness Lexus is not engineering the Autopilots on airplanes or we all we have been with a Pilot Deviation and to phone the tower....

 

My experience is similar as stated above on my 2016 RX. I prefer it that way, rather than expend additional wear on the brakes.

 

The system is functioning normally. It could be very dangerous for the vehicle to control speed using the brakes while going downhill. As you may imagine the breaks can overheat and fail if emergency or regular braking is needed.

 

Adaptive means it will adjust speed based on vehicle in front of you - but not hold a speed down a steep grade. We are still the captain on the bridge.

 

What I prefer is a system that will apply the brake to the speed limit when it senses a cop ahead.
No, I do not want the vehicle to hold down the brake on a downhill in cruise control if there's no car in front of it. Think of how hard it will be for the brake system and more importantly, the guy behind you will be cussing at you like no tomorrow.

 

I guess I understand the logic of what everyone is saying with brake over heating. But if you are going down a steep grade and the speed limit is 40.

Without using cruise: I would have to use the brakes to hold that speed manually braking.

With cruise: I would have to use the brakes once cruise control downshifting can't slow the car enough to keep at 40.

Either way I would have to use the brakes. I guess the cruise control also braked it would have less chance of overheating as it can both use downshifting and braking at the sometime. Maybe I am think headed but I feel either way the risk of brakes overheating are the same.

 

If there is a downhill descent for sustained period of time, my recommendation would be to put the stick in the manual mode and drive down on a low gear...

 

I haven't discovered this problem yet because I live in Houston, TX. (Can you say FLAT?) I wonder if taking it out of adaptive mode would do any better? I think you just hold the cruise activation switch in for a few secs and it switches modes.