mindstalk | best use of a street lane (Reply)

What is the most efficient transportation use of street space, e.g. in moving or serving people per hour? I've recently seen various infographics that had numbers, but no sources or calculations. Much better is this Urbanist article, which actually gives its assumptions and calculations. But I feel like doing my own estimates, though I will draw on that article, plus other research I've done. I'll mostly be analyzing a single 3-3.5 meter lane, in city conditions (lots of intersections, thus signalled to flow only half the time.)

cars: A safe-ish rule of thumb is 2 seconds following distance, so 1800 cars in an hour, halved for red lights, yielding 900 cars/hour. Average of 1.5 or 1.6 people per car; I want to be optimistic for cars here, so I'll round up to 1500 people/hour.

Arguably it should be less than that: one lane plus curbside parking will get clogged by people parking. A second lane might relieve that (cars can squeeze past a parker rather than being stuck), but also get clogged from weaving traffic. There's also the effect of left and right turns. But again, since I'm biased against cars, I want to make favorable assumptions for them, so we'll stick with 1500.

pedestrians: Contemplating a 3 meter span in my room, I feel 4 people could walk abreast; this PDF agrees, saying "a walkway width of 1.5 metres will comfortably allow two people to walk side by side". People are slow and stop easily, so one second following distance seems fair at max use (without an outright crush). That's 4 people/row * 3600 rows/hour = 14,400 people/hour, halved for red lights to 7200.

But note that if it weren't for cars, if this were a fully pedestrian space (car-free city, or just a wide underpass beneath intersections), even dense flows can interleave safely, and we could be be back at 14,400.

Also, if it were a full 3.5 meter lane, you might be able to squeeze 5 across, for 9000/18,000 people per hour.

In researching bike capacities, I learned there is another way estimates are done for signalized traffic: assume people are bunched up at a red light, and estimate how many can make it through in the next green light. Just visualizing a 1.5 meter sidewalk, it feels like 60 people could cross in 30 seconds (2 across, 30 seconds), thus 60/minute (1 minute clock time = 30 seconds green light + 30 red), and 3600/hour. Double that for being 3 meters wide, and we're at 7200/hour again.

bikes: This one is tricky, since I have little feel for dense bicycle flows or how they would intersect. Most conservatively, we could treat them as cars, except 2 across in a 3 meter lane, so 1800 bikes = 1800 people/hour (not counting child seats or tandem bikes.) This is already a bit more than the optimistic car assumption, plus generates no noise or pollution.

More aggressively, I can also envision 3 bikes across (especially in a 3.5 meter lane), with 2 bikes 1 second behind them, but offset so that everyone is 2 seconds behind the bike directly in front of them. That's 5 bikes per 2 seconds, 5*1800 = 9000 bikes, halved for red lights, 4500/hour.

The literature on bike capacity is rather varied. Lots of estimation, some measurement of actual routes. Lots of articles talking about saturation flow, i.e. how many bikes could go through if the light were green for a whole hour, and it's not always clear what's meant. A UC Davis study reportedly measured 2600 bikes/hour per 1 meter of lane. A Dutch estimate was 3000-3500 per 0.78 meters, based on headways. A Canadian test on a cycle track said 10,000 per 2.5 meters, or 4000 per meter of lane-width.

https://journals.sagepub.com/doi/pdf/10.1177/0361198119839976

"A case study in Beijing, China found 1,836–2,088 cyclists per hour for a 1.25 m-wide path (11). A 2 m-wide cycle-track in Santiago de Chile was reported to have a saturation flow of 4,657 cyclists per hour," (so, halve that -- also, 2 m wide.)

"Zhou et al. measured 2,500 bikes per meter per hour on cycle tracks in downtown Hangzhou."

https://sci-hub.ru/10.1080/23249935.2020.1859640

estimated capacity 2,000 to 2,500 bikes/hour/meter for c-bike (conventional bikes)

"Previous studies in China have suggested that the capacity of a bike lane is between 1,400 and 3,023 bikes per hour per metre"

"CDURE (2012) suggests that the capacity of a 1.0 m wide bike lane is between 1,600 and 1,800 bikes per hour if it is separated from the motorized traffic and between 1,400 and 1,600 bikes per hour if it is not separated"

So as you can see, it bounces around a fair bit. And I'm not sure if bike path capacity really scales with width, at saturation; I wouldn't want to be the middle biker of three. Probably a better bike use of a 3 or 3.5 meter lane would be making a bi-directional bike path. Still, it seems likely that at least a few thousand bikes/hour could be accommodated, maybe more.

buses: A standard 12-meter transit bus can commonly carry 80 people, as a mix of seats and standing. One of those every 5 minutes is 12 buses an hour, so 960 people/hour. Actually not as much as the car lane, though it should be a no-brainer to put bus service in traffic to increase capacity. But we can do better.

An articulated bus can carry 120 people, and allegedly 50 buses can run an hour before they get in each other's way badly, which leads to 6000 people/hour.

A bi-ariculated bus can carry 200, and thus 10,000 people. Real systems have been observed carrying 12,500/hour with one-lane surface street BRT, though possibly that's with crush loads. So a bus lane can definitely carry far more people than a car lane, if you use it right.

As a tangent, if BRT stations can take more width, like a loading lane and a passing lane, then the capacity can jump a lot, approaching metro levels. Imagine that stations have six loading positions, with buses able to pull and out independently (or pass if express.) 6 25-meter bi-articulated buses is 150 meters of vehicle and 1200 passengers, similar to a metro train. If a bus's dwell time is under 2 minutes, 3 buses a minute can be handled. 180/hour, so 180*200 = 36,000 people/hour. Whee! Plus, if you have a bottleneck station, some bus routes can simply skip the station, adding capacity to other parts of the network.

More conservatively (assuming short blocks, for instance, though unlike a train, you could have a cluster of bus platforms bracketing an intersection without blocking it), 2 platforms of bi-articulated buses enable 100/hour; 200*100 = 20,000 people/hour.

light-rail: Here I'm cribbing from the original Urbanist post I linked. 2 25-meter cars = 2*200 = 400 passengers; 24/hour has been done in the US, so 9600 passengers/hour. If blocks are long enough, you can run 3 or 4 cars. More conservatively, it's been said that light rail with signal priority should have 4 minutes between trains, so 400*15 = 6000 people/hour.

parking: This is trickier, since you don't have vehicles passing a point. I'll put my twist on the Urbanist approach. The US DOT says half of US car trips are under 3 miles. So let's say most are under 5 miles, or 8 km. The question is whether 8 km of curbside serves more people as parking spots, a bus lane, or what. Say there's 6 km of parking in that distance, and 5 meter-long spaces, so 1200 spaces. Say the average parking stay is 1 hour, so 1200 cars park an hour. 1.6 people per car, and 1920 people/hour. Quite likely less than a bike lane or BRT lane could serve.

Worse, Urbanist says that from a 2002 study, average parking time is more like 2 hours, so it's really only 960 people/hour served by parking. (Urbanist used different numbers, so gets a different result, but same ballpark.) Even a standard bus running every 5 minutes is competitive with that.

On the other hand, consider bike parking. Let's guess that the average bike ride is just 3 km. Using the same factors as before, that's 450 parking spaces -- but instead imagine they're dedicated to bike parking. Cambridge MA assumed that 14 bikes could use a bike stall they dropped in parking spaces in the summer; that looked crowded, so let's say 10. Assuming 2 hour parking stays again, that's 450*10/2 = 2250 people served/hour. Would those 2250 people prefer a bike path or abundant bike parking? Hard to guess.

So, there you have it. Granting that in modern society, we'll have at least one car lane on our streets, it's still a valid question as to whether an additional lane should be used for car movement or something else. And in fact almost any other use, except car parking, can serve more people. Probably the easiest approach is to split a lane's worth of space between a bike path and a wider sidewalk, but a bus lane or light rail track would be good as well. If you have two lanes' worth of space, at least at strategic places for BRT stops, then you can have really high BRT capacity.