The Wrong Way to Set Speed Limits [ST06]

Not Just Bikes
7 HN recommenders · recommended in 7 months, 2021–2025
YouTube
2.1M views · 5 years ago · explainer · evergreen

Not Just Bikes explains why posted speed limits and the 85th percentile method don't control how fast people drive. He argues that road design sets real speeds, and that streets need to be built so the safe speed is the natural one.

HN people cite it as the video that made them see street design, not rules, as what governs driver behavior.

“it really opened my eyes on the relationship between rules and actual behavior”
Urban design & public spaceDesign critique & analysis
#speedlimits#streetdesign#trafficsafety#85thpercentile#stroads#notjustbikes

15 comments from Hacker News

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@gregmac2024 · in “Bollards: Why and What”
Europe has a lot more roads with a lower design speed. Curves, narrow lanes, on-street parking, trees/poles/etc close to the road. These things cause people to drive slower, because it doesn't feel safe to go fast. In North America, roads are usually built in the complete opposite way, with long straight roads and wide lanes, so the design speed is actually quite high -- even if that wasn't the intent. People go fast, because it feels safe to go that speed, but isn't, because there are pedestrians and turns. We then "fix" that shit road design by having low speed limits. This video is all I think of when this discussion comes up now: ▶ this video
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n
@noirdujour2023 · in “The Style Guide for America’s Highways”
> MUTCD designs streets similarly to rural highways, and this can create conditions where people speed when they need to be driving slowly, because the design of the street doesn't reflect the speed the sign says you need to be driving. Not Just Bikes covers this shortfall in a video on speed limits[1], and it really opened my eyes on the relationship between rules and actual behavior. Essentially, speed limit signs affect driving speeds slightly, but the street design is important. Wide, open, straight roads naturally prompt drivers to drive to highway speeds, even in more suburb areas. It's amazing how often I check the speedometer and find that I'm driving much more than the posted speed limit when following the flow of traffic. [1] ▶ this video
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g
@gregmac2022 · in “New speed camera ‘detect’ vehicles braking as they approach”
I think my first intro to this was from a Not Just Bikes video on speed limits [1], and that profoundly affected my opinion on this stuff. I now think a lot of road design is just utterly broken, and it includes speed limits, what that channel (and other related) refer to as "stroads", and intersections (roundabouts vs traffic lights). That video in particular is highly relevant to the speed camera thing. [1] ▶ this video
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C
@CalRobert2024 · in “California Senate Passes Bill Requiring Passive Speed Limiters”
Not much but it’s a start. Unfortunately the US treats every street and road like it’s a country road when it comes to speed limits, with a lot of deaths as a result. Great video on this at ▶ this video
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g
@gcanyon2022 · in “The Exceptionally American Problem of Rising Roadway Deaths”
Not Just Bikes on YouTube covers road/pedestrian/bike safety extensively. link Specific videos: Crossing the street should be safer: link How to quickly build a cycling city: link Wasting money on car infrastructure: link The wrong way to set speed limits: ▶ this video Safe cycling: link "Stroads" are dangerous: link
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b
@ben-schaaf2022 · in “Heavier cars are safer for their drivers, but far deadlier for everyone else”
Not Just Bikes has a great video covering this: ▶ this video TLDW: People don't obsessively check their speedometer and thus changing the speed limit largely doesn't affect the speed people drive at. To effectively change that you need to change the road design.
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t
@tzury2021 · in “The Wrong Way to Set Speed Limits”
Submitted the video to Hacker News.
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E
@ErikVandeWater2021 · in “The Wrong Way to Set Speed Limits – The 85th Percentile”
Submitted the video to Hacker News.
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T
@TomK322025 · in “Making an intersection unsafe for pedestrians to save seconds for drivers”
The road design is what causes bad or good driving. The road you describe should narrow before the blind curve so the drivers would (often enough unconsciously) slow down before it. For the pedestrian crossing, small islands that separate the lanes and give pedestrians a safe space will help. ▶ this video
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C
@CalRobert2023 · in “Wales' 20mph speed limit: How has the first month gone?”
As much as the comments from complaining drivers are frustrating, it reveals that the problem here is relying on speed limits and not street design. If you want drivers to go 20mph, you need to design the street so going any faster feels uncomfortable. ▶ this video discuses this. Also, I notice that most of those drivers complaining don't address what it's like to have a small child who's a runner and is always trying to get loose of your grip and run in to the street.
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x
@xracy2022 · in “Speed traps have no long-term effect on speeding: study”
▶ this video This feels like doubling down on something that's not super effective. The way to make traffic slower, is to make road lanes narrower. You can also add curves/trees or overhangs to add shadows to the road, which breaks up the roadway and makes it harder to reason about from a distance.
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Y
@YawningAngel2022 · in “Even after $100B, self-driving cars are going nowhere”
That's not really how speed limits work though. People take their cues from road design and conditions, not what the speed limit actually is. Virtually changing speed limits is dangerous because it causes you to think you are altering the behaviour of traffic without actually doing so. ▶ this video goes into detail on this
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