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Comment by 01acheru | Hacker News Reader

There is one thing I will never understand, why do a lot of people talk about speed limits as the speed you need to be driving? It’s the maximum speed you can reach before being fined, it doesn’t mean “oh man I’m driving at 90km/h and the limit is 100, that’s so wrong”.

Some roads (at least here in Italy) also have a minimum speed requirement when there is no traffic, which goes higher the more you go to the left lanes, but that’s another story.

Anyway I think that reading speed limits from road signs is quite stupid and frankly useless, most of the time the speed you need to be driving is contextual and the same applies to a lot of other dynamics of driving.

Well done Mercedes, one step at a time raising the bar once you manage to do “simple” things right.

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It's not speed per se that is a problem, rather the aim of speed limits is to make sure you can control your car and not cause accidents.

If everyone is driving the 'recommended' speed of 130kmh on a German motorway then it is a severe risk if you drive slower. Road users need to be predictable. Trucks or motorhomes are slower, but you expect them to be slower. You see them from the distance and plan ahead. But a normal car that drives 60 when all others drive 130-140 is a massive danger as other drivers do not expect it to do so, so will lose precious seconds to react. You can get fined for driving unnecessarily slow.

In Germany you also cannot merge onto the motorway unless you have a certain speed already, and you cannot enter the motorway at all if your car can't do an appropriate minimum speed (or you have special transport papers and warning lights).

I didn’t know about the merging rule. Then why do they insist on such short merge lanes?

Do you happen to know the speed? I drive an Aygo (tiny, 3 cylinders) and I’d love to know how close I can get.

It's 60km/h to enter the motorway. You don't need to drive it necessarily (depending on your situation), but the top speed in your cars papers needs to be that or higher.

>If everyone is driving the 'recommended' speed of 130kmh on a German motorway then it is a severe risk if you drive slower.

So every truck on the roads is a severe risk?

Driving "too slow" is often dangerous as well. You'll get failed in a UK driving test for going too slow and not keeping up with traffic or generally causing an obstruction.

If you are doing 40mph but everyone else is doing 65-70 then you cause a hazard for people as they have to take evasive action to go around you, then others have to brake to avoid hitting the people going around you, then a ripple braking wave starts behind you, then a BMW driver rear-ends someone half a mile behind you and suddenly you've caused a multiple pile-up where someone died today, because you were going too slow for the conditions.

Uh, it is still their fault if they rear-end you.

That's a nice story that people keep telling because they don't like having to slow down.

But I seriously doubt that driving below the speed limit has a measurable increase in risk.

I would bet the opposite: That driving below the speed limit decreases risk of serious accidents.

One reason for that is that rear end accidents where both cars are going in the same direction are typically much less serious than head on collisions. So even if slow drivers increase the risk of rear end accidents (which I doubt is significant), they reduce the impact of head on collisions much more leading to a net positive.

People should drive slower, it's always safer. (Apart from dumb maneuvers like brake checking...)

Research from the 60s has a relative speed/crash incidence curve. It’s lowest for cars going a little above the median speed, about equally dangerous going either way from that in a U shape.

Some subsequent research affirms this curve, others call it into question. My own, informal read is that the stronger research generally affirms it.

It's not crash incidence I care about, it's "chance of being involved in a fatal accident" that I want to minimize.

Reducing the rate of harmless crashes is no good if the rate of severe crashes increases.

Speed-CAUSED crashes don’t factor strongly into fatal crash counts. Also, a general trend of increasing rural road or freeway (urban and rural) speed limit increases in the US since 1995 has accompanied continued plunges into record low highway fatality rates.

Also, all crashes are relevant. Fatalities can’t happen when there aren’t crashes.

The speeding crisis is mostly a moral panic.

It doesn't matter if the primary cause of a traffic accident is speed. The relevant question is:

"Would this person have died if the cars were driving slower?"

And simple physics tells us that almost always the answer is "No".

If you try to argue that driving faster is safer, you need to provide strong evidence. Statistical correlation over a time frame where cars and roads have been made much safer doesn't really convince me. Even a correlation between driver speed and collision risk is meaningless -- eg. people with poor vision generally drive slower, but are still more dangerous than drivers with good vision.

(I'm not saying that driving fast is unsafe in general. On a well maintained road with good visibility you can drive pretty fast with minimal risk. All I'm saying is that, all other things being equal, driving slower is almost always going to be safer)

I don’t disagree with the idea that if you’re going to get into a crash, that a slower-speed crash is usually less risky. Where I disagree is that we can stop at basic physics and ignore so many other factors. A focus on physics is kind of like the “but if it saves one life” argument, which is fallacious because it’s contemptuous of all other views.

It is not simply the case that anti-speed crusades result in safer roads. The USA’s 55 mph experiment a was a failure in that regard, and Vision Zero campaigns aren’t delivering on promises.

We really need to focus on crash elimination, which is not a hyper focus on speed. Crashes that don’t happen can’t injure or kill.

My main point was that we shouldn't tell people to drive faster. Every time an argument like this comes up, people on the internet start arguing that slow drivers are causing accidents. Open a random Reddit thread and it's full of people bashing slow drivers.

But I really doubt that slow drivers are inherently dangerous. Especially since a lot of road users are slow and can't drive fast (big trucks, tractors, mopeds, bicycles, RVs, cars with trailers, etc). So the focus must be on making sure that drivers are attentive and have good visibility, not telling slow drivers to step on the gas.

Slow drivers do increase crash risk if their slowness is done irresponsibly, which many of us see all too often. We all need to be courteous to each other, and driving slowly does not absolve one of that.

Sometimes, the right thing is for slow drivers to speed up and go with the flow.

If someone drives a little slower than average, it shouldn't be an issue. So I assume you are talking about really slow drivers.

If someone drives very slow, there's probably a reason why they are going so slow. It could be:

- they are distracted

- they are tired

- they are lost and don't know where to go

- they are intoxicated

- they don't see well at night or because of glare

- they are inexperienced

I don't think telling these people to go faster is going to make anyone safer.

Correct, and we’re missing these bad drivers since the basis of nearly every last traffic stop (yes, I’ve studied the data from several angles) is “speeding”. The further you lower limits, the more who are “speeding”.

> because they don't like having to slow down

An inability to read an argument without imputing your biases onto the person making it makes what you say a bit useless.

Here in Norway posted speed limit corresponds to safe speed at typical weather condition. If you go slower you impede traffic and force people to overtake you. That is substantially more dangerous than just driving at posted speed.

And Norwegians will overtake - I drove from Bergen to Oslo and was astounded by how many people would whip past me at breakneck speed in those mountain passageways. And I’m not what I would consider a slow driver. I chalked it up to inexperience with the area.

Huh. I drove Bergen to Oslo and averaged something insane like 47mph or whatever the limits required (since I was terrified of their speed cameras) and was rarely passed in 6-7 hours of driving.

In some countries you're supposed to drive at close to speed limit. If the speed limit is 90, you're supposed to go at least 70. Main reason why horse carriages and bicycles aren't allowed on highways. Go way slower than the traffic flow and you're impeding traffic which is detrimental to traffic safety.

In other jurisdictions, where it's not illegal, it's at least a reason enough for cops to pull you over, claiming "suspicious driving"

I failed driving school final test because I was too slow (not even on a highway)

not sure why you getting downed, for highways this is totally the case. In my country your vehicle must prove to be able to sustain at least 70km/h, or else it's banned from highways and high-speed motorways.

I think it’s because 70km/h is not “close to the speed limit” and also because there are countless reasons why you cannot drive a bicycle or a horse carriage on a highway besides the speed you travel at.

In that case would be far more useful for people to discuss about that than go all passive-aggressive-hn-downvoting style

It is close to the Czech highway speed limit for trucks, which drive on the same highway, so they must be taken into account.

> there are countless reasons why you cannot drive a bicycle [...] on a highway besides the speed you travel at.

What reasons other than speed are there for keeping bicycles off fast roads? Motorcycle can be very nearly just as small and vulnerable as bicycles, but they can keep pace with cars and consequently they're allowed on every road.

Bicycles are much more unstable than motorcycles, it’s more difficult for bicycles to avoid objects in some situations since you cannot brake and accelerate in a small timeframe, they take more time to return to a high speed after you brake, etc.

Same reasons why you cannot drive small motorcycles on highways here in Italy, even if they can easily reach 110km/h they are not allowed on even second tier highways.

I don't know what it's like in Italy, but in America bicycles are allowed on almost all roads, except for limited access highways. They're even allowed on most regular highways, to the chagrin of many car drivers. The poor acceleration of bicycles is the worst on roads where frequent stops are expected (for intersections) but bicycles are allowed on almost all of those roads, particularly city streets which obviously have abundant intersections. Limited access highways don't have intersections that would frustrate cyclists, but as a consequence of not having intersections those roads have much higher speeds that cyclists can't keep up with. Furthermore, heavy trucks have terrible acceleration, much worse than cyclists, but are allowed on all of the limited access highways that bicycles are banned on.

Maybe it's different in Italy, but in America the restrictions on bicycles really seem to be about top speed, not acceleration.

I get what you say, and sure laws are different from country to country.

Overall the reasons why you cannot drive bicycles on highways are many, but when you put them all together it mostly fall under one term: safety.

What I wanted to say on my original comment is that trying to put an autonomous driving car in a urban situation is not about checking the speed limit from a road sign, looking at a ground stop or other road signals. If you want a safe autonomous car in an urban center it must be aware of the context much more than “this is a stop sign” or “speed limit is 50km/h”, that’s why I praise Mercedes for trying to solve the solvable problems first and then move to more complex ones.

Road signs in Canada have provisions for minimum speeds, but I have not seen this posted anywhere I’ve driven.

My only source on this is a drivers training handbook.

In Canada you also are limited to driving according to road conditions. The posted speed limit could be entirely unsafe in winter.

> why do a lot of people talk about speed limits as the speed you need to be driving

In the UK you’ll fail you driving test if you drive below the speed limit, eg 60mph in a 70mph zone.

Hard to find an authoritative source, but this seems to break it down pretty well: https://www.drivejohnsons.co.uk/learning-centre/faq/driving-...

So, not quite. It’s that you can fail for “undue hesitation”, or driving dangerously, which could manifest as driving too slowly. But the act of driving to slowly isn’t a fail in and of itself.

'Undue hesitation' is something else (eg: failing to enter a roundabout when there's a safe opportunity to do so, especially if this causes other road users to also be unable to enter the roundabout).

'Appropriate speed' is definitely a potential minor (or major) error on your driving test (and you are marked as failing for either a single major or three or more minors in the same category).

Generally, it's up to the examiner's discretion whether speed is 'appropriate', but often lenience will not be more than 10% of the speed limit (unless there are other factors, such as weather, road surface, visibility, etc. which would make it unsafe to travel at the posted speed limit).

Driving at 60mph in a 70mph zone will not make you fail your test.

The maximum speed you are allowed to drive at is indeed, by definition, a limit. There is nothing wrong with the term.

That being said, driving laws, as far as I know, always say that you need to adapt your speed to the local conditions in order to drive safely, but you can never exceed the speed limit. Many (most?) jurisdictions have laws against dangerous and reckless driving and I expect it is possible to be found to be driving dangerously even when staying under the speed limit.

Where there is a minimum speed, which is also of course conditioned to current road conditions, it is also for safety. For instance, on a motorway where the traffic moves at 70mph it is dangerous for everyone if a vehicle decides to move at 30mph...

You should probably be driving quite close to the posted speed limit, unless there are weather or road conditions making that unsafe (then you should slow down), or if a lot of traffic is going significantly faster than that (then you should speed up). It’s not that odd to think that there should be one very narrow range of speeds that most traffic should be traveling at on a particular road and time, that’s pretty clearly the safest.

Nope, you should not speed up. It's a maximum. If others go faster, just ignore them and keep going.

In many states (including California) you will get a ticket for impeding the flow of traffic even if you are at or under the speed limit, e.g. if you are in the passing lane or not using turnout areas when on a two lane road. While you yourself don't have to speed, you do have to get out of the way of people who want to.

Ignoring everything that is happening around you, even if you are under the speed limit, is an incredibly unsafe way to drive.

German highways are special when it comes to this. Many of them have no speed limit at all. If traffic permits, you can travel at 180 mph and it's perfectly legal. You will see many Porsches doing so, you will see the electronically limited BMS do 155mph and you will see that things like vans travel at 100-110 mph as if it's a normal thing. This 60kph speed limit would simply look out of place on an Autobahn as they call it, and I gather this stem is only to be used on the motorway, specifically designated.

Sounds incredible until you witness it yourself.

It is. Imperative is “keep right”, and watch your rear view mirror as much as the road ahead. Doing 140mph and getting passed frequently is amazing.

German drivers are generally well-trained, and the mandatory car inspections means that cars are in good driving condition. The Autobahn is also in excellent condition compared to the Canadian highways I know.

I'd say that most cars drive at around 130-140 km/h in the center lane, and 170 km/h isn't unusual. This requires people to keep the left lane free, something I yet have to witness in North America.

But beyond that, the Autobahn is really boring to drive on. It's just like any other highway: almost straight, and completely isolated from the world around it.

> There is one thing I will never understand, why do a lot of people talk about speed limits as the speed you need to be driving? It’s the maximum speed you can reach before being fined, it doesn’t mean “oh man I’m driving at 90km/h and the limit is 100, that’s so wrong”.

Different traffic laws. In Germany, there's only a lower limit on the Autobahn, and there's an advisory speed limit that in precedent cases you must be able to maintain if your vehicle and road (and weather) conditions permit but in practice is somewhat overrun, which is legal.

Most states in the US penalize overspeeding, but also penalizes underspeeding on state and interstate highways if car, road and weather conditions otherwise permit. While counterintuitive, in highways (especially interstate ones) there's an expectation of a "belt" of vehicles that maintain more-or-less the same speed: different speeds of vehicles should be on different lanes (the innermost being the fastest lane otherwise posted in signage).

If you drove the limit on highways in Canada you'd be pushed off the road. People routinely drive 130 km/h+ on 100 km/h roads. Partly the fault of the engineers/province who designed them for those speeds and then tell people not to drive that fast.

If the posted speed limit is significantly lower than the natural speed drivers would drive or well below the actual average speed (both are often the case in the US), driving under the posted limit in clear/dry weather is more dangerous than driving at the posted limit.

Many of our highways are posted at 55 mph or 65 mph. Car traffic is typically 70-80 mph on those highways.

In the US you can and often will be pulled over for going too slow, because it’s suspicious. This effectively makes the speed limit the only speed you can drive without fear of police interaction.

I've had a few times in driving school where I'd interpret an end of speed limit wrong and go to the slower tier rather than the faster one. I got yelled at by the teacher on the passenger seat quite a bit. It's just not tolerated to go slower than the speed limit if there no good reason for it and it will lose you points in a driving exam, if not lose you the exam right away. At least that's how it is in France.

I was going to say the same thing. In France if the speed limit is 50 km/h, you have to go 50 km/h unless there is a good reason not too. Outside of cities, on the main roads the speed limit is usually either 80km/h or 90km/h. A few times while learning to drive with an instructor, he told I had to reach the max allowed speed limit as soon as possible as to not slow down the traffic and failing to do that will make me lose points on the driving test.

we were recently breathealyzed for driving 38 in a 50 zone. Even the police wants you to speed from one traffic light to the other

If the road conditions were good, then breathalyzing somebody who's going substantially below the speed limit for no apparent reason seems prudent. If road conditions were poor, then obviously that's another story.

If I remember correctly, back in the 70's in the US speed limits were lowered to increase fuel efficiency. I think people advocating for increased speed limits or no speed limits at all should at least consider the increase in fuel consumption and pollutant emissions when looking at the pros and cons.

It's because they're too low, that's why.

People are driving contextually, it's the limits that are not keeping up.

They're not blindly targeting the "speed limit" and throwing all caution to the wind.

(of course, they're ALSO often throwing all caution to the wind, but that's another story).

In the U.S. if you drive at (maximum) speed limit you’re likely slowing down traffic. You’re supposed to drive 5 to 10 mph above the speed limit.

Which is a bad thing. Especially since it gives cops the opportunity to pull over anyone whenever, because the expectation is that you speed all the time.

>You’re supposed to drive 5 to 10 mph above the speed limit.

What? Where is the legislation that says that?

Experience, not law.

My experience is if people are all driving at the same speed then they mentally switch off. When a hazard appears they aren't ready for it.

My car has a speed of 70mph on the SatNav when the speedo says 77.

That could be by design, or the factory fitted alloys having a different circumference to the factory steel wheels.

I peg my cruise control at 76.5 and overtake all day long because other road users are oblivious to their actual speed.

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