3 mph is almost 5km/h. If their technology is unable to stay within 3mph of a certain speed (which is laughable), they should drive 3mph slower than the speed limit.

In Toronto, if a sign says 60 (km/h), the driving instructor will tell you that you're supposed to go at about 65. In Sao Paulo, a 50 sign often means the radar is calibrated to ticket you at 50.1. We can't really make any judgment about being off a few miles unless we know what's the prevailing interpretation of speed signs in Tempe.

Besides, some commenters already pointed out that the google maps of the road in question says that the speed limit is actually 45 northbound (which was the direction the car was going), whereas the 35 number is the speed limit southbound.

I would be very surprised if your cruise control kept within 3mph of the set speed at all times in all conditions. My car certainly doesn't, and it's a 2017 model with a sophisticated adaptive cruise control.

I'll ignore ACC for a moment and talk about the modern implementation of regular cruise control, however. You'll have your target speed stored in a control unit, which cares about 3 to 4 things: your set speed, your current speed, the flow of fuel to the engine, and in newer models, access to the brakes. So, how do we determine the current speed? The drive shaft will have a magnet mounted on it, and each time it spins around there is a sensing coil that sends a signal to the control unit. Based on the frequency of these signals, it determines what your speed is, and compares it against your set speed. If it is low, it will send a signal to a servomotor that is connected to the accelerator, sending more fuel into the engine, hopefully increasing your speed. If it is going to fast, it will cut fuel, and on some models, apply breaks if you are more significantly over the set speed.

However, this is a pretty dumb system. It only knows how fast you want to go, how fast you're going, and whatever the formula is that it follows for increasing the flow of fuel to the engine. It doesn't know about millions of other factors that affect how fuel efficient that acceleration is going to be. It is constantly readjusting based on the new speed readings. To my knowledge, all manufacturers also go for something that provides a "smooth" driving experience for their adjustments, rather than how to most quickly achieve the set speed. When you take the limited data available for a cruise control system and combine it with built in smoothing to keep the ride comfortable, you end up in a situation where the cruise control is making frequent adjustments that end up on one side or the other of your target speed. The lack of perfect information means they purposefully do not attempt to perfectly match the target speed - attempting to due so would require so many adjustments that it would be pretty terrible for fuel economy.

Now, hopefully, the Uber vehicle is going to have even more data available than my ACC equipped car. But even with my ACC equipped car, almost all of the work has gone in better determining what the set speed should be, and significantly less on making sure it hits it exactly. I expect this is probably the same on the Uber vehicle. Because quite frankly, it's just not that important. To be quite blunt, the difference between being hit at 35mph and 38mph is probably just how far your corpse flies.

As for saying they should drive 3mph slower than the speed limit, then they might end up going 6mph under the speed limit instead of 3mph under.

I suppose my fundamental argument in the previous post and now this is: What outcome is expected from them being able to stay perfectly at 35mph instead of having enough variance to hit 38mph? What benefits are there? A few mph makes the most difference in damage from an injury standpoint at very low speeds, and these are the speeds that variance is almost always significantly lower - there's no evidence here that Uber cars are going 8mph when they're meant to be going 5mph, for example, and this would not be the type of variance we see in cruise control systems in general - and very little difference to the outcome at high speeds, where the greatest variance is likely to be shown.

That's a lot of text for explaining why a car might go to 3mph too fast. There is a big difference between a cruise control and an autonomous car. In Germany you can get a ticket for going more than 3km/h (1.875 mph) too fast. It's not acceptable for a self driving car to go so fast that you can get a ticket. If that means going up to 6mph too slow, I guess that's the way it is. Also, going 5km/h means you will need an extra 5m of braking distance or so in case of emergency braking. With electric cars, regulating the speed smoothly will be even easier.

It's an explanation of how cruise control works.

It sounds like cruise control is dangerous to use in Germany, based on your description. My 2012 VW GLI certainly did not stay within 3km/h of the set speed when utilizing cruise control.

>If that means going up to 6mph too slow, I guess that's the way it is.

If you want to be in a situation that is more dangerous than going 3mph over the limit, sure. If everyone was going 6mph under the limit this would be fine, but staying closer to the speed of traffic around you is less likely to cause accidents than going significantly slower. (Which, surely, if 3mph is significant, than 6mph is even more so)

>With electric cars, regulating the speed smoothly will be even easier.

I agree here. That just means solving the speed determination issue, which is not simple. GPS is the best method we have now, but is very problematic if there is no signal, or a degraded signal.

This would seem like a major problem then. In Silicon Valley it’s very common for freeway traffic to move at speeds that are 10 or 20 mph above posted limits. You either stay under the limit creating a dangerous situation as the result of differential speed or violate the limit and match the speed of surrounding traffic. How does the engineer not become liable in this context, like the VW engineers in the emissions scandle.

> It sounds like cruise control is dangerous to use in Germany, based on your description. My 2012 VW GLI certainly did not stay within 3km/h of the set speed when utilizing cruise control.

You don't need to stay within 3km/h of the speed limit to follow the law. You need to stay below 3km/h over the speed limit. So if your cruise control has a variance of 5km/h, you should set your cruise control at 2km/h below the speed limit.

> If you want to be in a situation that is more dangerous than going 3mph over the limit, sure. If everyone was going 6mph under the limit this would be fine, but staying closer to the speed of traffic around you is less likely to cause accidents than going significantly slower.

Unless your cruise control has a significantly larger variance than all of the other vehicles, you should be fine because everyone else should be setting their cruise control to roughly the same thing as you.

Sounds like maybe the culture is different in Germany for this. In the states 5 mph over the speed limit is normal. While you could get a ticket for it, you won't unless you're in a small town that's just trying to pad its budget. In some parts of the states even 10 mph over is regularly tolerated on highways.

I read your comment and thought that was a ridiculously tight margin of error, even for the Germans. Then I looked up the fine.

€15 for an in-town violation, €10 for out-of-town, no points for less than a 20 km/h variation? That's more of a secret tax than an actual deterrent.

Electric cars can't regulate the speed more smoothly than gas cars while on cruise control. It's about the control algorithms, not the source of power, and the gas systems could be on tighter control ranges if it weren't so inefficient and uncomfortable for the passengers. Adaptive cruise control runs on a PID loop control scheme and overshoot is inherently part of the game.

While on cruise control a car will go slightly faster than the speed setpoint for a few seconds, then slower, then faster, until it settles on exactly the correct speed. Then you go down a hill and it takes a little while to slow down, or you go up a hill and it takes a little while to speed up. Electric cars can use regenerative braking when the car is going down a hill but they're still constrained by the nature of control loops and aggressive braking will just lead to wonky acceleration-braking cycles while hunting for the setpoint.

That's also how humans drive. We just don't do it as well. If you're worried about tickets set your cruise for 3 km/h under the posted speed.

The setpoint is the setpoint. Key element of controls engineering. Secretly subtracting from the setpoint behind the scenes to compensate for your local driving laws would be the car lying to you and just leads to more trouble than it's worth.

Every day people tell me that they want a setpoint to be the temperature that the room never exceeds or the temperature it never falls under or five degrees above the highest temperature the boiler hits. It's like setting your clock ahead by five minutes to avoid being late. You can do it if you want to but it's ridiculous functionality to build into the timepiece.

Further, outside of detection errors like the one in this article, the reaction time of a self-driving car is orders of magnitudes faster than a human. You don't need 5m more braking distance per 5km/h because within milliseconds of the computer noticing the issue the car will hit the brakes. Human reaction time is more like 0.7s to 3s. You still need more time to brake as you go faster but the computer doesn't need as much time as our human laws already give us.

In a case like this where the victim enters the path of the car closer than the car's brakes are capable of stopping, even given instant detection, a human would have just hit the victim at a much faster speed because of the reaction time. That's going to happen sometimes. It's just how it works.

Basically everything you're saying is based on an outdated notion of how cars work and in particular your very German desire to follow the rules exactly. The rules are going to change, dude, and in the meantime you're free to set your cruise at 47 km/h to avoid accidentally triggering a photo-radar trap if you like.

> 3mph variance is perfectly normal even on modern cruise control.

That'd be a variance between true ground speed and the speed as estimated by the car. Chances are that the 38mph number came from the car itself, so there should be no "variance", leave alone 3mph over the limit. Modern cruise controls are perfectly capable of maintaining set speed, even at a considerable downhill.

> Chances are that the 38mph number came from the car itself, so there should be no "variance", leave alone 3mph over the limit.

My understanding, which may be incorrect, is that the 38mph number came from GPS recording, and that "cruise control" in Uber's self driving vehicles is not based on GPS speed, due to the impact poor/loss off GPS signal would impact it.

>Modern cruise controls are perfectly capable of maintaining set speed, even at a considerable downhill.

I'm not particularly handy with repairing cars, but I have a pretty decent understanding of how cruise control systems work due to nerding out on it when I bought a car with adaptive cruise control. Everything I have read on the design of cruise control says this simply isn't the case, because it isn't even a design goal. Much more important in the design is keeping close to it while maximizing fuel efficiency and driver comfort, and even in modern cruise control not all systems have access to the brakes, which would be required for keeping it at the set speed going considerably downhill. Anecdotally, all of the cars I have owned fit this and do not keep my vehicle at exactly the set speed, including the most recent one with ACC.

> Keeping to exactly the speed limit also just isn't how anyone drives.

People should not "keep to the speed limit". In England at least the speed limit is the absolute maximum speed at which you drive. You should be driving slower than the speed limit at all times, with no exceptions. Anything else is almost always incompetent driving.

I find it hard to understand why drivers think the laws don't apply to them. Sure, it's annoying when cyclists ignore road laws, but cyclists are far less likely to kill other people.

> In England at least the speed limit is the absolute maximum speed at which you drive.

In theory this may be true but it's absolutely not in practice. It's not uncommon to see people at least 20mph over the limit on motorways. On the M4 that is practically the norm.

Try sticking to the speed limit through long 40 or 50mph roadwork sections. I've done it for a laugh before. You end up getting overtaken non-stop including by trucks which, to me, feels far more dangerous than just fitting in with the prevailing speed.

I think there's something off about your arguments, human drivers speed, but they don't do it because of properties of the speedometer or the cruise control, or air resistance, or potholes - they do it because they don't care much about the speed limit.

It's completely possible to stay below the speed limit in a car, a lot of people just don't bother. Cruise control won't apply the brakes (at least in older cars) if you encounter a significant downhill gradient - that's up to the driver, and while speedometers are often out and GPS is more accurate, speedos universally over-read in my experience. If you're doing GPS 38mph your speedo is probably reading 40, which is not really within an excusable error margin of 35mph IMHO.

I would expect a fully autonomous vehicle to be able to accurately obey the speed limit and I suspect the reason it wasn't/doesn't is because that behaviour has been programmed in or developed. You could make arguments that doing what the other cars around you are doing is safer than being an outlier, but that's not the same argument you're making. I think the 38mph figure is significant.

Current systems cannot know the exact speed of a car due to the mechanics involved:

A speedometer's reading is measured on the gearbox, at the output but there are a myriad of little things after that including variance in the final drive, tire size and tire pressure all of which can make the "calculated" speed and the real speed to be within a few % of each other. All of these components are also affected by wear too.

At the moment you need a GPS to conclusively measure a car's speed with >99% accuracy but GPS signal isn't available/sufficient everywhere. One could argue that we should create some sort of highly accurate local system that measures speed and we certainly can but we'd be trying to solve a 0~10% speed measurement problem: we'd get no real value from it.

In other words: a car measures how fast a specific cog is rotating, adds a few variables and estimates how fast the car is going based on that.

edit: and all of this assumes the car is going straight, calculating speed when the car is curving is more complex.

>they do it because they don't care much about the speed limit.

Oh, certainly, and I apologize if my argument came across as "It's not possible for people to not speed" or "People only speed because of the properties of speedometers or cruise control"

My point is that speed within a few mph is already inaccurate, and that 3mph is pretty much within the current margin of error.

>speedos universally over-read in my experience

I don't know that there's a common leaning one way or the other in actual variance due to speedometer construction, but over-reading is the most common occurrence due to tire pressure being low causing it, and a huge amount of people drive with low tire pressure. Overinflated tires, or moving to larger tires in general, will result in under-reads.

>I would expect a fully autonomous vehicle to be able to accurately obey the speed limit and I suspect the reason it wasn't/doesn't is because that behaviour has been programmed in or developed.

I don't agree. lagadu covers this plenty well in his comment, however.

Speed isn't the issue, reasonable speed is the issue. As a human driver, if there are a lot of pedestrians - or I notice pedestrians stumbling/fumbling or seemingly not too conscious of where they're walking, I slow down.