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Adjustable Speed Limiter

69

Description

Although Tesla used the same stalk for cruise control as Mercedes, for some reason they only implemented half of it. The Mercedes cruise control can also be switched to speed limiter mode which sets a maximum speed rather than a minimum, actually providing a much easier to use (and safer) alternative to classic cruise control. It’s the one feature I would hate to give up when switching to a Tesla. Especially when driving faster than average traffic, using the limiter is much easier than constantly switching cruise control off/on whenever you have to slow down for traffic.

While cruising at a constant speed, the only difference between CC and limiter is the position of your foot. On cruise control, you tuck your foot somewhere away from the pedals (not exactly ideal for reaction time). On the limiter, you keep your foot resting on the accelerator (which is not fatiguing since you don’t have to modulate it, the weight of your foot is sufficient). In both cases the car behaves exactly the same: it simply modulates engine power to maintain the set speed.

The big difference is when you need to slow down for traffic.

On the limiter, you just ease your foot off the accelerator pedal to let the car slow down at whatever rate you choose, anticipating as much as you like. You then just follow traffic by modulating the accelerator as you normally would. When the road clears again, you press the pedal down to accelerate as smoothly or quickly as you like. As soon as you approach cruising speed again, the car smoothly reduces the acceleration and maintains the set speed (which you can always override, obviously). No need to switch anything off/on, no need to touch the stalk, real one pedal driving.

Compare this to cruise control: when there’s traffic, you either toggle the stalk several times to select some lower speed (which you then have to frequently adjust again as the traffic does not maintain a constant speed), or switch it off entirely. Deceleration rate is whatever the engineers chose for you and is relatively abrupt on most cars (I haven’t tried in a Tesla). Afterwards, when the road clears, you want to get back to cruising speed so you either hit the stalk several times again, or hit “resume”. The car now abruptly accelerates, again at whatever rate the engineers happen to have chosen for you, until you reach cruising speed. One pedal driving? No, one foot going on and off the pedal plus lots of left hand toggling and an uncomfortable experience for passengers.

On the limiter, I can drive hundreds of miles in and out of traffic without hitting any levers or touching the brake. With cruise control, you’re constantly adjusting it or switching it off and on. I was recently forced to use a classic cruise control and the constant toggling drove me nuts.

There are also a few safety issues where the limiter is, in my opinion, safer than cruise control:

1. Since you have to switch off cruise control when slowing down for traffic (or constantly modulate the speed up/down), you may end up distractedly following that traffic even as it slowly accelerates above your normal cruising speed without you noticing it. This way, I have actually ended up driving 180 km/h instead of my normal 160 km/h cruising speed (which is already well above the legal limit). 160 is just a fine (I like to call it a speed tax), but 180 is an immediate driver’s license revocation in my country. With the limiter engaged, there’s no such risk. You keep following the car in front in a natural way until it gets too fast, at which point the limiter automatically keeps you from absentmindedly speeding more than you intended.

2. Switching off cruise control or selecting a lower speed is a deliberate action with a slightly higher psychological threshold than simply easing your foot off the accelerator. When driving a car with cruise control, I often find myself going faster than I should in certain situations (cars in front doing funny things, bends in the road)  because one tends to hesitate a bit before switching off CC or hitting the brakes. When cruising on the limiter, it’s much more natural to just ease off on the pedal a little bit and gently slow down. Then, when everything is safe again, you smoothy accelerate withouth ever having had to hit a switch or move your foot to a different pedal. Whenever something on the road makes me uncomfortable, I often notice that I had already slowed down without realizing it. Not so on cruise control, where I would still have been cruising at the same speed. This is probably the reason why we even have road signs warning people to switch off CC when approaching road works.

Competitive/Pricing/Notes

On Mercedes cars, the tip of the stalk is used to switch between cruise control and limiter. The yellow light next to “LIM” comes on when the limiter has been selected. Tesla has assigned a different function to the tip and the light, and removed the letters “LIM” (even though they are still displayed in the photo in the manual last time I checked, and probably present in early models), so I would suggest adding the option to the touchscreen instead. Just an extra toggle switch in the menu: “Cruise Control” / “Speed limiter”. This should be a simple software update.

Status

Unknown.



Category: Tags: entered 22-Jul-2014

11 Comments

New idea that makes a lot of practical sense ...

Adaptive Cruise Control is of course a step up if it can handle the need to automatically slow down ... but for situations where ACC isn't working or isn't available for any reason, this preset max speed idea is a good one
The described Mercedes cruise control sounds fabulous but still has its negative points.  I've never heard of CC functioning this way.  Can the driver manually increase speed over the top set point with just pedal input or are they limited to the top set point until they either set it higher or cancel it altogether?  Depending on the answer this is a negative IMO.  Regardless, the extra feature is a personal choice and one I do not think I would be likely to take advantage of.

I finally took delivery of my S85 in early March.  My driving environment is consistent.  The roads and traffic are suburban and very predictable.  While I have no need for a speed limiter I can intuitively understand its usefulness.  Nevertheless Tesla's Traffic Aware Cruise Control (TACC) has my admiration.  This has been my very first experience with TACC.  It is taking a little while to become accustomed to all of its behaviors but I am working through them.

The one behavior I still do not like, but understand fully, is the very conservative behavior of slowing down longer than a normal human would for the vehicle ahead turning right or left, i.e., the Tesla is still on the brake even though the vehicle ahead has completed the turn for about 1.5 - 2 seconds and is obviously moving away at a ~90 degree angle.  I understand this behavior because when I was a young driver I accidently caused a slow speed rear end collision assuming the vehicle in front of me was going to complete their turn in normal fashion.  You never know what the driver of the vehicle in front of you will do.  When you think about it much of driving is based on assumptions.  Most of the time drivers maneuver their vehicles based on assumptions gained over the course of their driving experiences.  Most of the time is not the same as 100% of the time.  Once I determined I have no way of knowing what causes the Tesla to react this way or the ability to change it programmatically I came to accept it.  Now, when I find myself in traffic on uncontrolled access roads I simply switch off the TACC.

The Tesla TACC on controlled access highways is quite capable in my short time of ownership.  I took an extended trip recently and the TACC performed as expected which allowed me to squirm around in my seat in order to change positions during extended periods of sitting without needing to pay any attention to the speed or following distance.  Insofar as speeding up over the TACC set point, this is a manual maneuver IMO and one I used effectively a few times during my trip.  I did not find it difficult to moderate my top speed because I realized I was back in manual control of the vehicle albeit for top speed and guidance only.

 
C'est un must  as we would say. that was my first question when i drove the Tesla for the first time.

it allows you to accelerate without looking to the speed limit a real value with this funny and fantastic accelerating car!!!
Nothing on the car delivers this benefit.

 

A speed limiter is simple and safe plus it protects your licence and delivers responsible reputation for the tesla drivers.
I thought I might be able to accept TACC as a good alternative, but from what I've read, it reacts too slowly (taking 3 seconds or so before accelerating when the car in front has moved to the right), often brakes late (instead of anticipating and smoothly coasting to the speed of the preceding traffic), follows swerving motorcycles without noticing the stationary car in front, accelerates either too slowly or too abruptly, etcetera. And if you ever heard of defensive driving (especially important for agressive drivers like me), well, TACC doesn't have a clue.

So I really wish that limiter arrives before I finally buy my Tesla...

(still waiting for more range to handle my rather long commute)
There is no doubt that TACC is a very smart bit of kit. The downside, for me, is that when there is no car in front, it accelerates to the set speed. On a country road, say 40mph or even 50 mph, it might be legal to drive at that speed, but often, in some areas, it is more sensible to be driving at a slower rate. SpeedLimiter allows you to do so. If you do need more speed, to accelerate out of a situation, you just floor the pedal and it overrides the limiter. So, yes you can override it. Yes it does allow you to drive at whatever speed you want below the set limit. The overspeed warning does provide an alternative by pinging as you speed past the limit you have set, which should alert you, just in time, to look up and see the mobile speed camera flash you as you get a ticket. The ping is helpful, but not as helpful as something that stops you speeding in the first place. Apart from Autopilot, I regard this as THE most important driver aid - particularly with the MS, as it speeds up SO quickly. I want to be looking out, at the road ahead, not looking in to see if I am at, or past, the speed limit.
That's quite true, indeed. TACC will slow down for traffic ahead, but not for the drunk swerving in the right lane, the cars on the entry ramp that want to get in, the tight bend up ahead, the stretch of bad road with puddles you know is up ahead, etcetera. A speed limiter allows you to regulate your speed and really adapt it to the situation, while letting you effortlessly cruise at your chosen speed when the road is clear. And most importantly, going back and forth between cruising and slower stretches without having to hit a single button or the brake pedal.
Anyone who has had a car with Speed Limiter, knows that there are NO downsides. All it does is limit your top speed. You still have to drive the car, you still have to accelerate and decelerate, braking does not switch it off (unlike CC). If you want, or need, to go faster, flooring the pedal achieves it. It allows you to accelerate safe in the knowledge that you are NEVER going to break the speed limit, get caught by sneaky mobile speed traps, and can enjoy all the excitement of driving. AND, at the end of the day, it is NOT compulsory. If you don't want to use, you don't have to select it.
Have it and had it on all of my BMW's, and I'm really missing it on my 'S'

This function can be combined (and improved regarding to the one from Mercedes, BMW and others) with the feature request from another post for an auto speed limit based on GPS location.

The comments in that other post regarding safety issues when overtaking a truck, are not taking into account that you can always overrule the limit, simply by making a cickdown of the accelerator pedal.
I'm apprehensive that an aging driver like myself who's accustomed to CC and never tried a speed limiter, might react wrong when initiating braking or passing when switching between the two different systems. But I wanna try.

To enable CC, while driving press down all the way on the shift stalk. How about enabling speed limiter by bumping it up all the way, disabling cruise?
How could you "react wrong"? When the speed limiter is on, it just keeps you from absentmindedly going faster than you intended. When you release the accelerator, the car slows down. When you brake, the car brakes. When you accelerate again, it again keeps you from going over your set limit (unless you override it by flooring the accelerator or turning the limiter off). It's really intuitive.