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The problem with that approach is that it only considers drivers, and it lets the aggressive drivers influence the speed limit by bullying safer drivers. Outside of freeways, we shouldn’t dedicate millions of dollars in public space to private car drivers, so we should be looking at the rest of the community and ensuring that cities aren’t built around the idea that everyone needs a private vehicle. That’s a financial hardship for many people (where I live, cars are neck and neck with food as a household expense, behind only housing) and it freezes out the roughly ⅓ of the population who can’t drive. It also locks in significant health and environmental costs and we don’t tax driving to pay for those.

See the Solomon Curve - https://en.wikipedia.org/wiki/Solomon_curve

We absolutely should consider the entirety of drivers and their existing behaviors when we design roads, instead of wishing they would behave differently.

Moreover, reducing accommodation for motor vehicles in cities leaves the non-urban population behind - and neglects that private vehicles aren't the only users of the roadway. Truck deliveries, garbage collection, and mail become a lot more difficult if roadways are reduced.

Note that I’m not saying we don’t consider drivers at all - only that we look at the needs of the entire city rather than saying anyone who doesn’t drive doesn’t matter or, especially, that people who live there are less important than faster commutes for non-residents as was the case for much of the 20th century.

> Moreover, reducing accommodation for motor vehicles in cities leaves the non-urban population behind - and neglects that private vehicles aren't the only users of the roadway. Truck deliveries, garbage collection, and mail become a lot more difficult if roadways are reduced.

Non-urban drivers shouldn’t have primacy over residents just because they chose to live further away and refuse to use transit. Moreover, there’s a fundamental flaw in the assumption that speed limits are the primary limiting factor there: we actually know from decades of studies that they’re slowed by other cars, and adding lanes tends to make things worse by incentivizing more car trips. Boston famously spent enormous amounts of money on the Big Dig - which the MBTA is still struggling to pay off - and the net impact was a slight time savings for suburban commuters for about six months, after which traffic was worse than before because more people were driving. Getting those people onto trains would be far more effective and cheaper.

Similarly, delivery and garbage trucks aren’t affected at all by calming measures because those drivers aren’t flooring it all the way downtown - they’re stopping too often. What does help is reducing congestion and simplifying roads because that makes their routes more predictable (they don’t have the option of picking an alternate route) – if we wanted to help them, we’d ban unprotected left turns or reserve one parking spot per block for deliveries.

It sounds like you're working with a base assumption that increasing usage of cars is bad.

My base assumption is that moving people around is good. Freedom of movement and time-efficient movement. The car reigns supreme over trains, planes, and buses in those terms.

> the net impact was a slight time savings for suburban commuters for about six months, after which traffic was worse than before because more people were driving.

By the sound of it, more people were able to get where they were going because of the the Big Dig.

> Getting those people onto trains would be far more effective and cheaper

A breakdown of the numbers here would be great, but I'm not sure how much more time either of us want to spend lol. I like trains, but I'm cautious about them because they generally can't take people directly to an end destination the way an independent vehicle can. The US has also generally failed to use rail in a way that could replace cars in a meaningful way, despite rail having a decades-long head-start over the automobile.

> Similarly, delivery and garbage trucks aren’t affected at all by calming measures

I was under the (perhaps mistaken) impression that you wanted to remove or replace roads in cities to make way for alternative transportation. That's based on the statement in your original comment:

> Outside of freeways, we shouldn’t dedicate millions of dollars in public space to private car drivers,

I was not referring to traffic calming measures when I said that would be a logistical nightmare. I more meant that private cars share common infrastructure with a number of critical services; reducing their share of public space would harm those other services.

Look, I grew up in California suburbs where car culture is the only culture, but I’ve also looked at the data enough to know that the status quo isn’t working. It’s clear that cars do not scale even if you spend amounts of money we can’t sustain, they have significant negative impacts on people who live near busy roads, and the planet isn’t exactly getting any cooler.

More to the point of the article, we know that every year forty thousand Americans are killed by unsafe operation of vehicles and at least hundreds of thousands are injured, usually with permanent life altering impacts. Experts estimate that there is close to a trillion dollar annual gap between what drivers pay and the cost to society for the status quo: https://www.economist.com/united-states/2024/01/18/why-car-i...

So the question from my perspective is how we improve quality of life for everyone while reducing costs for everyone, and that means less car trips which also improves the car trips which are necessary. For example, emergency vehicles are most commonly blocked by private cars. If we care about improvement there, getting more people to walk, bike, or take transit helps everyone because a fire truck or ambulance isn’t routinely stuck in traffic. The same applies to safety for children, the ability for the blind or elderly to function independently, ease of delivery, etc.: every single part of urban life improves if there isn’t the assumption that everyone in the city needs ~120 square feet of space to move themselves around, and the heavy equipment is reserved for the small percentage of trips which actually need it.

> Look, I grew up in California suburbs where car culture is the only culture, but I’ve also looked at the data enough to know that the status quo isn’t working.

Then you would also be 100% aware your desire for people to walk and take public transit is entirely at odds with reality. It does not work in suburbs or rural areas. The only place these ideas work in reality is in dense urban areas...

First, you’ll note that I specified urban a lot. Transit definitely doesn’t work for everyone but it’ll work where the majority of the population is.

Second, transit not working in most American suburbs is an artificial choice. Transit works well in the railroad suburb I live in now but the white flight-era suburbs were intentionally designed not to work for transit or pedestrian access and while later designs might have been less intentional about it, low density is a challenge. That low density is also what makes them financially unstable and is creating challenges for an aging population, so I wouldn’t treat it as evidence of strong demand rather than people picking the most heavily subsidized option. California has had a huge boom in ADU construction when it was streamlined, revealing a great deal of market demand for density which was previously stymied.

When you work 40+ miles away from your house - no, suburban (and urban) public transit simply do not work. It is a fiction public transit can solve transportation problems in the US.

Interesting perspective. Enjoyed the discussion.

Differences in speed matter, but so does absolute speed.

"Inappropriate speed is responsible for 20 to 30% of all fatal road crashes." from https://www.itf-oecd.org/speed-crash-risk

Also, due to kinetic energy increasing with speed, collisions are worse, in addition to being more likely.

> Get the average speed, add 5, and post the sign.

A common method for setting speed limits in the past was the 85th percentile speed.

I agree that changes to road design are a good idea when a significant number of people drive too fast, with "too fast" being determined by risk factors that are not necessarily obvious to drivers. Adding curb extensions, chicanes, or narrowing lanes by painting new lines are some inexpensive options for encouraging slower driving.

I'm against nannies built into cars or heavy punitive enforcement.

Optimistically, this rule could create tighter feedback loops due to increased community engagement with planners so that databases get updated more quickly and speed limits are perhaps dialed in to match real conditions--the mismatched limits that you mention might have been established with expectation of a neighborhood buildout that never happened, or may be an oversight in low population areas--or they may be there for good reasons.

As it stands, speed limits are not usually enforced and are basically ignored by drivers, so (optimistically) this may encourage communities to make them more realistic limits, as opposed to suggestions.

> There is a road near me (SWFL) where everyone routinely goes about 50 MPH, despite a 35MPH limit.

You say this but in SoCal I see vastly many more people driving 35 in a 50 than 50 in a 35. Which is... Probably what causes a lot of the speeding problems in the first place and is, frankly, vastly more dangerous because people set expectations around the signs.

Rust/deterioration will take care of that.