Understanding traffic
13 comments
·November 3, 2025Straw
pessimizer
This will only affect poor people. Rich people will continue driving everywhere they want as if it didn't exist.
bobthepanda
There is a floor to this; there are people so poor they can't afford the ongoing expense of a car at all.
Straw
At high demand times, you have to be very rich indeed to outbid a full bus without even thinking about it. There aren't enough people who can do that.
But say this does happen a lot-this means rich people pay enormous road use fees, which can then be used for road maintenance, construction, and improvement, as well as other transit infrastructure!
So, the rich willingly subsidize infrastructure for everyone? Seems like a win-win!
yesfitz
The same can be said of any tax meant to curb a behavior (sin tax).
What traffic-reducing policy would you suggest such that all people are affected equally?
AlgorithmicTime
[dead]
Noumenon72
> If a straight stretch of road has 4 intersections with stop lights for cross traffic, and one of those lights is green for 20 seconds for the straight road and green for 40 seconds for the cross traffic, then the end-to-end throughput of that road (ignoring turns on/off for the sake of simplicity) is 1/3 of its hourly capacity, or 600 cars per hour. Widening the road won’t fix that intersection.
I don't see how the intersection affects road-widening calculations at all. Doubling the lanes will double the throughput, to 1200 cars per hour. We weren't expecting widening the road to also eliminate red lights.
bobthepanda
is the relationship between lanes and throughput linear? even where it's illegal people will change lanes and do all sorts of suboptimal things with the additional space; particularly if people need to shift multiple lanes to be in the correct legal lane.
dr2chase
author here, you are right, I missed that. In my pathetic defense, the normal argument around here (Cambridge, MA) is about literal lane widening and narrowing, and not adding and subtracting.
IcyWindows
I don't understand the bicycle density numbers in the article.
At high speeds, bicycles also have to spread out. Add the bike trailers mentioned, and it seems even more unlikely.
dr2chase
Hi, author of the article. I'm assuming urban traffic speeds, which is what I observe all the time myself, but you can look at the video of those kids, and count, and look at the seconds. 125 bikes in 45 seconds, between 0:02 and 0:47. Understanding it is another issue, but it's a fact. (This is one of those things that I do myself and would not claim that I exactly understand the details, I just do it.)
There have been more academic studies. e.g. https://nacto.org/wp-content/uploads/5_Zhou-Xu-Wang-and-Shen... estimates 2512 bicycles per hour per meter of road width, or 7536 bikes per hour on a 3-meter (10 feet) wide lane. That's only 4.2x car throughput, versus those kids who managed 5.5x.
You are right about the trailers, but at least where I ride, they are not common-case for carrying things, lots more cargo bikes instead, and those are "better" than trailers -- it's possible to ride two cargo bikes side-by-side even in a US protected lane (specifically on Garden Street in Cambridge, MA), though this of course assumes competent riders.
01HNNWZ0MV43FF
> Car throughput is maximized at around 30-35mph
That's funny. That means that the interstates are optimized for speed, not throughput. I believe it, it's just counter-intuitive.
Noumenon72
Optimizing them for speed makes them flexible: when they're not full, you can go fast, and when they're full, they can degrade gracefully to 30-35 mph.
This doesn't mention the most economically sound and complete solution to traffic: dynamic congestion pricing on roads.
Due to the effects described in the article, entering a road that's close to congested imposes negative externalities due to the delay on everyone behind you, even higher if you are pushing the road below optimal throughput. Push that externality into the price, and suddenly drivers will change their behavior in the desired fashion:
1. People will move their travel to less expensive times. Even if no other change occurs than people waiting for prices to fall, the roads operate at much higher throughput due to never getting into the region of diminishing throughput.
2. People will carpool/vanpool/mass transit- no need for any special treatment for transit, a bus with 50+ people can simply outbid most cars on the road for space, even accounting for the difference in road space taken by the bus. With the economic incentive in place, you'd even expect private buses/etc to pop up spontaneously. Right now, its rarely worth it to pool/bus- it adds extra time for you, but the benefit to the road you never see. With proper pricing, its still faster to take a car, but a lot more expensive- and the carpool/bus/etc is still probably faster than driving would be with congested roads.
3. Similarly, the high prices will incentivize alternatives such as biking, subways, etc, and give very good information on exactly what routes are in high demand when, estimates of how much an improvement would be worth, etc.