[HN Gopher] Understanding traffic
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Understanding traffic
Author : kunley
Score : 20 points
Date : 2025-11-03 11:55 UTC (4 days ago)
(HTM) web link (dr2chase.wordpress.com)
(TXT) w3m dump (dr2chase.wordpress.com)
| 01HNNWZ0MV43FF wrote:
| > 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 wrote:
| 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.
| Noumenon72 wrote:
| > 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.
| dr2chase wrote:
| 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.
| bobthepanda wrote:
| 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.
| IcyWindows wrote:
| 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 wrote:
| 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.
| Straw wrote:
| 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.
| pessimizer wrote:
| This will only affect poor people. Rich people will continue
| driving everywhere they want as if it didn't exist.
| Straw wrote:
| 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 wrote:
| 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?
| bobthepanda wrote:
| There is a floor to this; there are people so poor they can't
| afford the ongoing expense of a car at all.
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