Would this be considered good UX if the car in safety assist mode would drop the speed automatically to adjust with the car in front instead of informing the driver of getting too close?

This is currently used by Tesla in autopilot mode which is not really an "assist" as driver is taking very little to no part in controlling the vehicle.

In my opinion this would become vital in preventing rear collision, but could also affect drivers who are not used to this.

2 Answers 2


I agree with Mani kv, however the full answer was hidden in Mercedes S-Class that has already got the Active Break Assist. Video can be seen here.

ABA vs Human (source)

Most drivers, under normal braking conditions as well as under emergency conditions, start out with little brake pressure and whenever necessary they will increase their pedal effort. In an emergency this behavior leads many times to a crash since the car could not be stopped in time. Those situations require maximum pedal pressure from the beginning - if necessary the effort can be reduced later in the process.

I addition to that, I forgot to consider the reaction ability.

A controlled study in 2000 (IEA2000_ABS51.pdf) found average driver reaction brake time to be 2.3 seconds.

After reading this study I think there is no doubt that dropping speed automatically is a win-win. In addition to that it can always be switched off in case, where driver wants to act "bad boy".

  • 1
    When the user wants to act like a bad boy the on board computer (probably named HAL 9000) should say "I'm sorry Dave, I'm afraid I can't do that for you". Feb 9, 2018 at 14:09
  • I guess for this to actually feel good for the driver the detection would have to begin a lot further than that. The braking behavior in the video looks horrible, every time there's a pedestrian it makes a hard stop, shaking the driver around. That's probably due to it being a prototype but I wonder if detection from further away leads to new problems.
    – Big_Chair
    Feb 9, 2018 at 14:22
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    @RobEarle, you made me lough! Feb 9, 2018 at 14:45
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    @Big_Char, If this technology will save a life, I am okay with being shaken for a little bit. Feb 9, 2018 at 14:45

In my view, I think its a good UX, since from the user's perspective he/she expects the car to behave that way. For instance if I put my car in autopilot, I want the car to take decisions for me and only notify me for some critical decisions or interventions. Since slowing down car is relatively common while driving, this can be perceived as annoying/non-functional if users are prompted every time the car needs to slow down. Plus, its safer if the car takes control over this since its hard for users to jump suddenly into the situation and take control to slow down or the car will crash.

Hope this helps!

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