科学与数学priceonomics.com
所有限速都太低了吗？
Is Every Speed Limit Too Low?
2014 — 2017，它一次次回到首页
黑点是上首页（≥ 50 分）的那次投稿，空心点是没冲上去的投稿，点越大分数越高。
导读
这篇文章探讨美国限速标志的设定方法，质疑许多限速是否定得过低。文章介绍了'85分位车速'原则——即限速应设在85%的司机实际行驶速度附近——并指出道路设计本身（车道宽度、弯道半径等）才是真正决定司机开多快的因素，而非限速牌。
为什么经久不衰：它揭示了限速与实际车速脱节的系统性原因，涉及交通工程、执法与道路安全的经典争论，多年后仍是城市规划讨论中的常青话题。
HN 评论精选
限速讨论不能只考虑司机，车速直接决定行人的伤亡率
这种分析只有在假设路上只有汽车时才成立，但现实并非如此。车速对行人等弱势道路使用者被撞后的伤亡率影响巨大：碰撞速度16英里/小时时行人重伤风险约10%，31英里/小时达50%，46英里/小时达90%；死亡风险在23英里/小时约10%，42英里/小时达50%。风险还因年龄差异很大，例如70岁老人被25英里/小时的车撞的风险，相当于30岁的人被35英里/小时的车撞。讨论这类问题时请考虑路上所有人，而不只是司机。
mcfunk查看英文原文
This is a fine analysis if you imagine that every road only ever has cars on it. But this is not the reality, and speed of cars has a huge impact on the fatality rates of vulnerable road users when they are hit. Results show that the average risk of severe injury for a pedestrian struck by a vehicle reaches 10% at an impact speed of 16 mph, 25% at 23 mph, 50% at 31 mph, 75% at 39 mph, and 90% at 46 mph. The average risk of death for a pedestrian reaches 10% at an impact speed of 23 mph, 25% at 32 mph, 50% at 42 mph, 75% at 50 mph, and 90% at 58 mph. Risks vary significantly by age. For example, the average risk of severe injury or death for a 70‐year‐old pedestrian struck by a car travelling at 25 mph is similar to the risk for a 30‐year‐old pedestrian struck at 35 mph. https://www.aaafoundation.org/sites/default/files/2011PedestrianRiskVsSpeed.pdf Please think of everyone on the roads, not just drivers, when making arguments like these.
道路设计而非限速牌决定了实际车速，应从设计入手
限速牌与实际车速相关性弱的一大原因是：人们开多快取决于道路设计而非标志。交通工程标准根据预期车速规定车道宽度、弯道半径、护栏等，且内置了安全余量——比如45英里/小时不失控需要200英尺弯道半径，实际会按55英里/小时设计成300英尺。结果人们按'安全'的速度开车而非限速牌。解决方案似乎是按更低速度设计道路、缩小安全余量或增加减速措施，但这能否兼顾安全尚不确定。
ranger207查看英文原文
A big reason that the posted speed limit is only loosely correlated with the actual speed driven is because the speed people go is not based on the sign, but rather on the design of the road. There are various standards for traffic engineering that define various parts of the road based on the speed expected on them: things like lane widths, curve radii, the presence of fences and guards, and more. A 35mph road will have a smaller turn radius than a 65mph highway, for example. These standards have margins built in to them, obstinately because the standards for faster roads are safer. If, for example, you need a 200 foot turn radius to avoid skidding at 45mph, then the road will be designed with a 300 foot turn radius that would prevent skidding at 55mph, and a 55mph road will have a 400 foot turn radius that would prevent skidding at 65mph. However, what happens is that people will drive at the speed that's safe, not the speed that's posted. People will go 55mph around the 300 foot radius turn that's posted for 45mph. The solution to this seems to be to make the roads designed for lower speeds, or to reduce the safety margin, or as other commenters suggest to add traffic calming measures. I'm pretty sure this would reduce traffic speeds, but I don't know if it would reduce traffic safety.
文章缺乏学术依据，车速与事故率的关系有扎实研究支撑
这篇文章无视了规划、公共卫生和土木工程领域大量同行评审文献，反而依赖一位警官的经验之谈。一个合理的思维模型是：车速同时增加事故的频率和严重程度。文章确实隐含了一个我认同的观点——道路设计需与限速配套，宽车道和大退线会鼓励超速。但应该是设计服从政策，而不是政策迁就过去糟糕的设计。文章'我们都在超速却很少见到车祸'的说法与同行评审研究相矛盾。
peatmoss查看英文原文
This article ignores a lot of peer-reviewed literature from planning, public health / injury prevention, and civil engineering in favor of some folksy wisdom of a lieutenant cop. A good starting mental model is that vehicle speeds increase both the frequency and severity of crashes. The article weakly, and quietly made an argument that I do agree with. Namely, roadway design needs to accompany speed limits to ensure people actually drive the desired speeds. Wide lanes, big setbacks, homogenous environments encourage speeding regardless of what is posted. But let's be clear, we should have our design follow our policy, not our policy follow bad designs that were put in place by roadway engineers of yesteryear. That message seemed to be lost in the race to tell us something tantalizingly counterintuitive. I love "everything we know is wrong" storytelling as much as the next guy, but only when that's actually backed up with solid evidence. Take for example this gem from the article: > “We all speed, yet months and months usually pass between us seeing a crash,” Lt. Megge tells us when we call to discuss speed limits. “That tells me that most of us are adequate, safe, reasonable drivers. Speeding and traffic safety have a small correlation.” That would seem to be at odds with this peer reviewed article (sorry, paywall, but you can read the abstract): > Respectively, they found evidence for an exponential function and a power function between speed and crash rate. Both types of studies found evidence that crash rate increases faster with an increase in speed on minor roads than on major roads. At a more detailed level, lane width, junction density, and traffic flow were found to interact with the speed–crash rate relation. Link to article: http://www.sciencedirect.com/science/article/pii/S0001457505001247 Priceonomics's banner ad tells me it is selling a new book titled "Everything is Bullshit." Quite right.
一个真实案例证明司机依据道路设计线索开车，而非限速牌
我学过结构工程，至少在加拿大，道路设计考虑的是司机应对突发状况的能力，而非人们实际开多快。有个有趣的观察：一段标为60km/h的高速路，大家都开95km/h，偶尔警察开罚单也没用。后来发现是立牌人看错了数字，实际限速本应是90km/h——护栏、车道宽度、路肩、匝道坡度等设计线索让司机开出了真正'预期'的速度，完全无视限速牌。
3pt14159查看英文原文
I studied Structural Engineering in University, which was under the Civil program, and, at least in Canada, we don't design for what people will go, but for how drivers can handle unexpected conditions, like cars turning out in front of them, a loss of traction due to ice, and visibility at night. I've also seen multiple studies that show that during high visibility / traction days (summer during the daytime) the speed limit can be safely raised to at least 130km/h for most highways that are 100km/h. One interesting thing I observed while driving to university was that there was this one section of highway that was marked at 60km/h, but everybody drove around 95km/h. Every now and then there would be a cop that would give out a bunch of tickets, but it didn't seem to change the speed people went. Well it turned out that the person that ordered the sign had a dyslexic moment, and the speed limit was later changed to 90km/h. What was interesting was that the design of highway (the guards, the lane width, the shoulder, the banking of the ramps) were all cues for drivers and they drove the actual intended speed without regard to the maximum speed sign. I think that's neat.
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