The Wrong Way to Set Speed Limits [ST06]

Speed limits are important to keep our roads and streets safe but how are they decided? And what happens when a speed limit is set too high or too low? There are right and wrong ways to set our speed limits, and design our streets, but unfortunately North American traffic planners are firmly stuck in the 20th century when it comes to building safe streets.

To learn more about what's wrong with North American traffic engineering, check out the new book by Strong Towns founder Charles Marhon, "Confessions of a Recoving Engineer":

NJB Live (my bicycle livestream channel):

---
References & Additional Reading

Understanding the 85th Percentile Speed
Strong Towns

ACCIDENTS ON MAIN RURAL HIGHWAYS RELATED TO SPEED, DRIVER, and VEHICLE

Road safety - Speed
World Health Organization [WHO]

DRIVING SPEEDS AND PEDESTRIAN SAFETY

Literature review of pedestrian fatality risk as a function of car impact speed

Paris passe à 30 km/h lundi : les habitants de la capitale plutôt favorables

Gezicht in de Carnegiedreef te Utrecht. [1969]

---
Chapters
0:00 Intro
0:07 Introduction to the 85th Percentile Speed
1:24 Streets vs. Roads
2:10 Where the 85th Percentile might make sense (roads)
2:38 Where the 85th Percentile doesn't make sense (streets)
3:44 Momentum kills
4:40 Signs are not enough (driving is subconscious)
5:49 Bad speed limits (city streets)
6:39 Bad speed limits (rural streets)
7:32 Confessions of a Recovering Engineer
7:52 How to make drivers slow down
8:18 Good slow street design
9:05 Safe speeds by design
9:35 OK Boomer
9:55 Conclusion
10:36 Patreon Shout-out
10:48 Outro

[slomorenga]

In southern Italy they have a fantastic and economic way of slowing cars down: they just don’t maintain the streets at all and let potholes grow everywhere. So drivers are in constant fear of damaging even their SUVs. Works perfectly.

[jumpingfreak]

Being a European and having played American Truck Simulator, I kept getting confused by the constant speed limit change, despite literally nothing else changing with the road. It feels so incredibly arbitrary.

[SebastianTheGreat]

Civil Engineer here. The “sending your lowest paid intern out with a clipboard” part is hilarious and 100% what happens every time our traffic department needs to do a traffic study

[xanadian]

We had a highway/traffic engineering professor at UMaine who taught this exact approach. This was back in the 90s. And, probably to the surprise of nobody, he was from Sweden.

[JoonasD]

"This approach seems reasonable, until you think about it at all."

This is amazing and widely applicable.

[mvz]

The amount of common sense dismissed due to perceived sunken costs and bad habits of old school traffic design is unbelievable. Your entire channel is telling people very basic realities and solutions and yet I get a fresh perspective on something every time. Love it.

[LiiMuRi]

In France and other places in Europe, when you come from a country road with 80 km/h limit to a village with 50 or 30 km/h limit, they often have various kinds of bends, chicanes, dividers and poles forcing you to slow down. Very effective

[bryanmilstid]

I have to admit, I had zero interest in any of these topics until I came across your channel. Now I'm buying books and looking in to how I can help affect change. This is honestly the best series and channel that I have had the pleasure of viewing. You make the subject come to life and have relevance to your viewers. Keep up the excellent work.

[davislinkaits]

I'm quite amazed by the design of the signs themselves. If I were the driver in Canada, I would surely miss some 90% of the speed limits. They don't pop out and are put in corners. In contrast, every country in Europe uses a prohibitory round-shaped sign with a red outline to indicate a speed limit. They, for most of the time, stick out and are placed in easily accessible places.

Maybe this is another thing that the US and Canada need to look in to.

[Flugmorph]

Had no idea traffic designers would choose the speed limit based on the average speed of observed drivers on that piece of road.
that is absolutely insane to me.

[Lazirus]

The "design speed" of the road is such a good point you bring up. We have a road that enters into our neighborhood. The lanes are wide, it's completely straight and there isn't an intersection or anything until 3/4 a mile down. The speed limit is 30mph and it feels agonizing to drive down that road every time.

[aaronbritt]

We had a road in my city that we thought had a short section with a speed limit set artificially low. We found that a speed limit study had been done and that the limit, by the 85th percentile rule, should be that same as the rest of the road. We asked the city raise the limit at a city council meeting and they refused. We then found that the city had been raising a significant amount of money through civil fines for speeding on that one section of road. We sued the city and won. We forced them to raise the limit to the same limit as the rest of the road.

[pavarottiaardvark]

What's REALLY clever about the trees at 8:58 is that the firs four are spaced out and then they get closer together. This makes the driver feel like they are going faster than they are, and encourages them to slow down.

[grandbuba]

"This method seems reasonable. Until you think about it. At all".

I've been having this realization about a lot of things lately, it seems.. :-)

[CharlieND]

5:24 "most people don't consciously think about what speed they're going"

Then there's me who checks my speedometer every five seconds because I'm paranoid about being pulled over for no reason

[junahn]

I suspect that one of the big reasons why there is no incentive to change these bad streets/roads/stroads is that many municipalities rely on the revenue collected from speeding tickets.

[alexriddles]

The best road sign I ever saw was at the edge of a small town in Missouri. It didn't say speed limit 35. It said the lights are timed for 35. I immediately slowed to 35 at the edge of town, stopped at one red light in the center of town and drove through about 10 other traffic lights all of them green.

[erikalexander]

I keep getting sucked into these mini-documenteries (or whatever the correct name is) even though I have nothing invested in the topics I keep watching them and finding them highly interesting and educational. Great work :)

[joshualogue]

Officer: Sir do you know how fast you were going?

Me: Lemme tell you 'bout a little thing about the Psychology of Road Design

[BlahBlahBlah]

In Australia, we tried a rule that you must slow down to 40km/h (24mph) if there are any emergency vehicles with flashing lights on the side of the road - for example, a police car giving someone a speeding ticket. The rule even applied on 110km/h highways (86mph). There was extreme pushback but the government trialed the rule anyway. The 1-year trial was ended early due to the sharp increase in rear-end collisions and videos of semi-trailers slamming their breaks to match the changed speed. In some cases you had one policeman writing someone a ticket and another in the same car with a speed gun out to catch those who didn't slow to 40km/h in time. It was a fucking mess.