What is the “Correct” Speed Limit?

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There’s a trend towards lower speed limits in cities all over the world, but why is this happening? What is the research behind it? And what is the “correct” speed limit for cities, anyway?

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References & Further Reading

Confessions of a Recovering Engineer
Charles “Chuck” Marohn
https://www.confessions.engineer/

SWOV Fact sheet – The relation between speed and crashes
https://safety.fhwa.dot.gov/speedmgt/ref_mats/fhwasa1304/Resources3/08%20-%20The%20Relation%20Between%20Speed%20and%20Crashes.pdf

Rosén, E., Stigson, H. & Sander, U. (2011). Literature review of pedestrian fatality risk as a function of
car impact speed. In: Accident Analysis and Prevention, vol. 43, nr. 1, p. 25-33.
https://pubmed.ncbi.nlm.nih.gov/21094293/

NACTO – Impact Speed and a Pedestrian’s Risk of Severe Injury or Death
https://nacto.org/wp-content/uploads/2017/11/2011PedestrianRiskVsSpeed.pdf

Nilsson, G. (1982). The effects of speed limits on traffic accidents in Sweden. In: Proceedings of the international symposium on the effects of speed limits on traffic accidents and transport energy use, 6-8 October 1981, Dublin. OECD, Paris, p. 1-8.

Kinetic energy management in road traffic injury prevention: a call for action
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4288294/

Stopping Distance Calculator
https://www.omnicalculator.com/physics/stopping-distance

New crash tests show modest speed increases can have deadly consequences – IIHS News

Speed and the Laws of Physics
Société de l’assurance automobile du Québec
https://web.archive.org/web/20221108022042/https://saaq.gouv.qc.ca/en/road-safety/behaviours/speed/laws-physics

Shaffer, D.M., Maynor, A.B. & Roy, W.L. The visual perception of lines on the road. Perception & Psychophysics 70, 1571–1580 (2008). https://doi.org/10.3758/PP.70.8.1571

Driver Reaction Time
Marc Green – Human Factors & Science
https://www.visualexpert.com/Resources/reactiontime.html

The Effect of Driving Speed on Driver’s Visual Attention: Experimental Investigation
https://link.springer.com/chapter/10.1007/978-3-319-07515-0_18

The Critical Ten – Stgreets.mn

The Critical Ten

Zhang, W., & Peterson, M. (2011). Predicting Patterns of Potential Driver Distraction Through Analysis of Eye-Tracking Data.
http://onlinepubs.trb.org/onlinepubs/conferences/2011/RSS/program.pdf

Why Speeding – Just a Bit – Can Be Dangerous
ARCON Forensic Engineers
https://arconforensics.com/system/ckeditor/attachment_files/184/the_dangers_of_speeding.pdf

Serious Injuries 2018 – European Road Safety Observatory
https://road-safety.transport.ec.europa.eu/system/files/2021-07/ersosynthesis2018-seriousinjuries.pdf

Svenson, Ola (February 1981). “Are We All Less Risky and More Skillful Than Our Fellow Drivers?” (PDF). Acta Psychologica. 47 (2): 143–148. doi:10.1016/0001-6918(81)90005-6.

McCormick, I. A.; Walkey, F. H.; Green, D. E. (June 1986). “Comparative Perceptions of Driver Ability: A Confirmation and Expansion”. Accident Analysis & Prevention. 18 (3): 205–208. doi:10.1016/0001-4575(86)90004-7. PMID 3730094.

E. Verheijen and J. Jabben, Effect of electric cars on traffic noise and safety,
RIVM report 680300009/2010, http://www.rivm.nl/bibliotheek/rapporten/680300009.pdf

Alexander RG, Macknik SL, Martinez-Conde S. Microsaccades in Applied Environments: Real-World Applications of Fixational Eye Movement Measurements. J Eye Mov Res. 2020 May 15;12(6):10.16910/jemr.12.6.15. doi: 10.16910/jemr.12.6.15. PMID: 33828760; PMCID: PMC7962687.

Slowing city traffic cut road deaths by a quarter
https://www.ed.ac.uk/news/2022/slowing-city-traffic-cut-road-deaths-by-a-quarter

Too Fast, Too Furious: New York City’s Speeding Epidemic
https://transalt.org/reports-list/too-fast-too-furious-new-york-citys-speeding-epidemic-and-the-case-for-local-control-of-speed-limits

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Chapters

0:00 Intro
0:45 False Start NJB Logo
0:51 Real NJB Logo
0:56 Why 30km/h?
1:33 Noise
2:27 Kinetic Energy
3:33 Mass
4:24 Microcars
5:08 Walking & Cycling
6:15 Avoiding Crashes
7:39 Visual Perception
9:00 SMIDSY
9:29 Overconfidence
9:55 “That guy”
10:20 Mobility vs Safety
11:07 Faster and Slower
12:25 Too Much Traffic?
13:07 Brainerd Traffic Lights
14:50 Removing Traffic Lights
15:56 Painfully Inefficient Stroads
16:32 Building Great Streets
17:30 Fuel Efficiency
18:33 The Benefits
19:47 Conclusion & Assholes
20:20 Ground News

Amsterdam made a big change recently near the end of last year these covered signs started appearing all over the city then on a foggy December morning Christmas came early as literally overnight all of the stickers were removed to reveal that these were now 30 km hour streets the majority of streets in Amsterdam were 30 km hour already but these roads had a 50 km limit before and these were the ones that changed to 30 this now means that the majority of the city has a 30 kmph speed limit this trend towards lower speed limits is happening in cities all over the world so why is this happening what prompted these changes what is the research behind it and ultimately what is the correct speed limit for cities anyway it’s 30 but sometimes lower there are a lot of good reasons to lower speed limits Within cities but the largest and most obvious benefit is safety especially for anybody outside of a car a person hit by a car going 30 km per hour has about a 95% chance of surviving the crash but the fatality rate increases sharply with speed in any reasonable world this chart alone would provide more than enough justification to reduce speed limits within cities unfortunately we do not live in a reasonable world we live in a world where convenience for drivers is routine rely prioritized ahead of safety so let’s continue while safety is by far the most important reason to lower speed limits Amsterdam also Justified this change because of noise the noise from Cars comes from two major sources propulsion noise created by the car’s engine exhaust transmission and brakes and rolling noise which is caused by the tire’s friction with the road surface the rolling noise gets louder the faster the car goes and it starts to exceed the propulsion noise at about 30 kmph in fact at above about 50 km/ hour electric cars are just as loud as gasoline cars so sorry switching to Electric doesn’t magically solve this problem there is significant environmental physical and psychological harm that is caused by the noise of Motor Vehicles and I have a previous video about noise if you’d like to learn more Amsterdam expects traffic noise to reduce by 1/ half on these new 30 kmh streets but let’s get back to safety you know what kinetic energy is right if not your high school physics teacher is very upset with you right now you get it from this formula notice that the V is squared so it goes up very quickly as velocity increases this means that even a very small increase in speed can result in a huge increase in the energy of a crash as you can see in this video by the Insurance Institute for Highway Safety modern vehicles have a lot of safety features that are designed to dissipate that energy into deforming the vehicle in instead of deforming the people inside but and I know this is hard for many suburbanites to understand there are people who exist outside of cars of course the best solution is to never have to dissipate so much energy in the first place and the best way to do that is lowering speed limits changing a street from 50 to 30 kmph means reducing the energy of a potential crash by 2/3 that’s absolutely massive and this is why there’s such a big difference in fatal crash rates at 30 versus 50 kmph notice also that the other element of that kinetic energy formula is mass and unfortunately there has been a trend for vehicles to get larger and heavier over time see my video but these stupid trucks to learn more in the 1980s it wouldn’t be unusual to find a family driving say a Ford Fiesta but today you’ll literally see idiots commuting to their desk jobs with some ridiculous thing like a Dodge Ram 00 which can be up to four times heavier the kinetic energy of that 80s Fiesta going 60 kmph is the same as a vehicle four times its weight going 30 kmph so it’s perfectly reasonable for yesterday’s 50 or 60 km perh roads to become today’s 30 kmh roads to compensate for this trend towards larger heavier vehicles and when speeds are slower and there’s less energy in a crash fewer safety features are needed too for example most airbags don’t even deploy unless the impact speed after breaking is about 30 km per hour this allows City cars to be even smaller lighter and more compact without sacrificing safety it should be safe for people to drive a micro car like this around the city because if you can’t even get around safely in a little metal box then it’s even less safe to be outside of a metal box which is totally unacceptable because then the city is unsafe for anybody who can’t drive like literally every child for example there are beneficial knock-on effects to this too when cars go slower the streets are safer and when the streets are safer people are more likely to walk or cycle on high-speed roads people cycling need to be physically protected from motor vehicle traffic with protected bicycle paths like this one but even here in the Netherlands streets with a 30 kmh or lower speed limit do not require any bicycle infrastructure at all well provided that car volumes are not too high a good example of this is in Tokyo where a lot of people cycle despite having almost no protected bicycle Lanes this is because the city has so many low-speed streets that people can use to cycle to their destinations there are countless benefits to having more people walk and cycle from public health to a reduction in traffic congestion and when the streets are smaller and slower less infrastructure is needed in general enal which reduces infrastructure and maintenance cost too if you’d like to learn more about the benefits of walking and cycling as well as those financial implications then check out the YouTube channel not just bikes I’ve watched every one of his videos multiple times they’re great of course the safest car crash is the one that doesn’t happen at all and the slower everybody is moving the more Reaction Time everybody has to avoid a collision the faster a car is driving the longer it takes to bring it to a stop I really hope that’s obvious to everyone but if not your high school physics teacher is even more upset with you braking distance varies greatly between Vehicles a heavier SUV will normally have a longer braking distance than a smaller car though this can be offset with larger brakes but regardless on a given vehicle the stopping distance at 50 kmph is about double that of 30 km an hour and the stopping distance doubles again between 50 and 80 kmph and that’s a problem but in order to stop in the first place the driver needs to react to what’s happening in a best case scenario when a driver is paying attention and expecting to stop a typical reaction time is about 700 milliseconds but for an unexpected stop like a child running out into the street that driver needs time to process what they’ve seen and the reaction time is more like 1.5 seconds here’s how far a car can travel in that time and this is before the brakes are even engaged this extra 10 to 15 M or more can easily be crucial to avoiding a fatal crash with someone outside of a car so in a busy City where lots of unexpected things might happen it makes sense to have much lower speed limits our brains take huge shortcuts when processing visual information and this is incredibly important when it comes to driving because we don’t see nearly as well as we think we do when driving on the highway the lines on the road seem very short right but in reality they’re is long as a car we tend to think of our vision like it’s a camera seeing everything as it happens but that is absolutely not the case our eyes and visual cortex evolve to process what we see at about the speed we can run so strange things start to happen when we move faster in order to keep a clear view of the world in front of us our eyes need to refresh what we see regularly and this happens through what are called catic eye movements that happen subconsciously two to four times per second researchers have used eye tracking Hardware to prove that the faster we drive the more our eye movements tend towards the center of our field of vision this may not be much of a problem when traveling 30 km hour but this is an estimate of what we see when traveling 50 km/ hour and this is 80 km/ hour you’re really just seeing part of your visual field at any one time and your brain fills in the rest so this is what you think you see when you’re driving but this is what you actually see and your brain does a lot of work to make it look like a complete scene just about every motorcyclist is familiar with the concept of smidy it’s when a driver almost runs you over and says sorry mate I didn’t see you this is often attributed to bad or inattentive drivers and that definitely does happen but it’s not the entire story if the driver’s eyes didn’t happen to look at that part of their visual field for a few hundred micros seconds it is literally possible that the image of the motorcyclist was never actually perceived by that driver and the majority of people overestimate their ability to drive too multiple Studies have shown that about 80% of people believe that they are an above average driver this false sense of confidence along with a lack of understanding of basic physics results in people driving at dangerously high speeds which might be fine if they were only endangering themselves but they’re not they’re also endangering everybody else especially anybody outside of a car oh now okay I’m really sorry but I can already tell there’s some guy writing that comment that always shows up in discussions of lower speed limits if 30’s safer why not slower let’s go zero then everyone’s safe Mom when’s dinner wow nobody has ever said that before you are so clever now go sit in the corner and try not to eat paste while the adults finish their video obviously there is a trade-off between mobility and safety but one look at traffic fatality statistics and it is blatantly obvious that we are well well on the unsafe part of that balance going back to the graph of a person’s chance of fatality you can see that multiple Studies have been done that show a slightly different curve but the inflection point is always at around 30 so this should be our starting point but to be honest I don’t like how only deaths are so often talked about when it comes to Road Safety it’s estimated that for every road death there are about 10 times as many people with serious injuries many of which result in lifelong disability now maybe I’m crazy but I generally think it’s a good idea not to permanently disable people to save a few minutes on your commute while 30 is a good starting point there are situations where lower speed limits are Justified too this neighborhood in Amsterdam has been completely redesigned recently and the recommended speed limit is now 20 kmph this is reasonable for a small residential neighborhood like this with a lot of foot traffic and of course there are roads where higher speed limits are perfectly reasonable too a limited access road with no people walking or cycling on it can easily have a speed limit of 50 kmph or more for example Amsterdam has allowed many public transit Lanes to keep their 50 kmh speed limits which is why this sign has this exception the logic here is that public transit Vehicles driven by professional drivers in a dedicated Center running Lane can safely travel at higher speeds I mean there isn’t really a good reason to make trams drive slower just because cars are dangerous they’ve indicated this via these signs on the ground to make it clear to anybody walking or cycling that Vehicles may be traveling faster here and of course highways are built with wide Lanes clear shoulders merging lanes and gradual turns to allow relatively safe driving at much higher speeds too but when you drive into the City and there’s a lot of activity such as parallel parking turning cars and lots of people walking and cycling then you can slow down for the safety of everyone so are there any downsides to 30 kmh speed limits with within cities well no not really the typical response is that it will grind traffic to a halt now again I don’t have a lot of patience for this argument one side is effectively saying I want to drive fast everywhere and the other side is saying I want people not to die on the street so honestly I don’t even know why we entertain this discussion as far as I’m concerned one person’s convenient should never Trump another person’s safety but regardless this line of thinking is wrong anyway faster speed limits do not always mean faster or more efficient traffic within a city it’s a lot more complicated than just big number go fast there’s a chapter in Chuck marrone’s book confessions of a recovering engineer it’s a genuinely great book you should read it Link in the description where he talks about commuting in his hometown of Brainard Minnesota now Brainard is a small town of less than 15,000 people downtown looks like this there’s absolutely no reason why a town as small as brainer needs a stro this wide running through the middle of it of course but that’s not important right now this strad has a 35 mph speed limit but when truck measured his actual speed limit through this area every day he averaged about 10 mph which is 16 km per hour for everyone else and that’s because well okay why don’t you explain it Chuck hey Jason not just bikes welcome to brainer what you’re getting here is our main intersection in the center of town and yeah if you’re driving through here you might Hit the Lights just right you might be able to drive 40 mil an hour through the middle of the city um but the odds are you’re going to hit at least one red probably more than one and in that case you’re going to sit and go zero for an extended period of time and if you look at the average speed you spend a lot of time going zero and while you might get to go 40 m an hour for a little bit of time the average speed is ridiculously [Music] low of course these traffic lights are necessary because high-speed intersections are dangerous by comparison roads with a 30 kmh speed limit do not really need a lot of traffic lights it’s perfectly safe for drivers to just merge out of a side street and yield to any traffic on the road it’s so safe you can even go out backwards okay now you’re just showing off and where two 30 km roads or streets intersect one can have priority over the other with yield signs or drivers can yield to the right which is common in many cities in in Europe This is possible because at slow speeds there’s lots of time for everyone to react of course roundabouts are also possible but at high speeds round of boats are quite dangerous for people walking and cycling especially if they have multiple lanes and stop signs suck and they should almost never be used but I have a previous video about that Amsterdam has been lowering speed limits and reducing car volumes for years and because of this the city has been able to remove many traffic lights such as here at this intersection here everybody follows the mark workings on the road these triangles are effectively yield signs and if they’re pointed towards you then you need to yield I’ve stood and watched this Junction many times and it works very well and traffic of all kinds tends to flow better when there are no traffic lights but this is only possible because speeds are low if these streets were still 50 kmph this couldn’t be done safely and One Direction would always need to be waiting at a traffic light at all times similarly when turning onto a high-speed stroke like this one in my hometown it can take quite a long time to find a suitable Gap in traffic this is extremely common when turning out of a driveway like this and this can also be a dangerous turn especially if a driver rushes into a small Gap in traffic for a busy shopping center what inevitably happens is that turning traffic gets so backed up that Traffic Engineers decide to install another traffic light which is just one more traffic light that through traffic might need to stop at lowering the average speed of travel this is the reason reason why strods are so uniquely inefficient but fundamentally traffic volume should be very very far down the priority list for city streets anyway if you’re talking about a highway or other limited access road then it makes sense to talk about traffic volumes but on a street the speed of car traffic is nearly irrelevant there are so many other metrics that should be looked at first when designing a city street such as how safe it is how comfortable it is to walk there how to provide the most foot traffic for businesses how to provide trees and Greenery how to provide spaces for children to play how to deliver public transit and how to make it accessible to people with disabilities how trucks will load and unload for shops and how to allow for Community interaction and even how to make it aesthetically pleasing so that people want to be there and when Mobility is concerned the priority should be about moving as many people as possible not as many cars as possible and that is done by providing space for dedicated Transit lanes and making it safe and convenient for people to walk or cycle if there able to the other common argument against lower speed limits is fuel efficiency many gasoline cars have a sweet spot at around 60 kmph where it’s fast enough that the engine works most efficiently but not so fast that wind resistance becomes an issue and okay that is a thing but I’ve also noticed that drivers suddenly seem to care a whole lot about the pollution caused by their automobiles but only when they’re not allowed to drive whatever they want wherever they want and as fast as they want like if you read really and truly cared about the pollution from your car you would drive a more fuel efficient car or you’d take the bus but you don’t right but the rest of us do actually care about the pollution from your car so the best solution to that is to have fewer cars in the city for example we could put Park and Ride garages for your car on the edge of town and make it easier to walk cycle and take public transit plus that frees up space on the road for the trucks and vans that we actually need in the city anyway so let’s do that instead then we can have cleaner air and fewer dead people on the road winwin it’s still early to know exactly how beneficial the switch will be in Amsterdam as data is still being collected anecdotally I’ve noticed that most drivers are driving slower though there are exceptions of course when the data is in I’ll be making a follow-up video on how Amsterdam lowered the speed limits in most of the city and how they’re not just putting up signs but also redesigning several roads so subscribe if you’d like to see that and if you can’t wait for that video then check out my live streaming Channel NJB live where I’ll be visiting several of these streets and bringing a radar gun to see exactly how fast people are driving I have no doubt that Amsterdam will see huge benefits from lower speed limits because we already have data from several other cities Brussels saw a 25% decrease in people seriously injured or killed on streets that were converted to 30 km per hour in Edinburgh collisions Dro by 40% fatalities by 23% and serious injuries by 33% on streets converted to 20 mph and residents also reported an improvement in their quality of life and that’s important too New York City tried to reduce speed limits to 20 mph but had to compromise on 25 because well America but even then traffic fatalities fell by more than 22% and pedestrian fatalities by more than 25% the thing that bothers me is that the data is really really really clear here at some point we have to realize that anybody fighting against lower speed limits within cities is either will ignorant or they’re a selfish [ __ ] who values their convenience more than other people’s safety and I mean [ __ ] those people seriously I am so tired of our cities being loud uncomfortable and dangerous because of [ __ ] who drive too fast and think the rules don’t apply to them we need to take back our streets and lowering speed limits is a big step in that direction unfortunately there is a lot of misinformation being purposefully spread to stop cities from lowering speed limits and much of that comes due to pressure from the automobile insurance and fossil fuel Industries who are some of the biggest owners and advertisers to news sites which is why it’s important to understand the bias behind your news and a great way to do that is with this video sponsor ground news ground news is a website and app designed to give readers an easy datadriven way to read the news every story comes with a clear breakdown of the political bias factuality and ownership of the sources reporting all backed by ratings from three Independent News monitoring organizations if we look at that story in ground news you can see that there’s a lot of extra information provided in a neat and organized way and you can see how different news sites report on the same story the trend details are here along the right hand side nine news sites reported on this story and you can see that the majority of the coverage was in the center you can also see the factuality and ownership information for this story a third of news outlets are owned by individual billionaires and another third are owned own by media conglomerates it’s also great that ground news lets you compare headlines to see if there’s a bias in the framing notice that only one of these articles frames this change as a quote Monumental waste of time while the others are much more positive another feature I really like is the blind spot feed which highlights stories that are disproportionately covered by one side of the political Spectrum for example if you’re only looking at right-leaning media you probably miss this story about how the EU is negotiating a 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