So in the first post I said "I remember that back in Albany, my peak traffic counts were on Marin or San Pablo, about 10.3 cars per lane-minute."

But I remembered something key last night: left-turn lanes. Both streets had them. Traffic on Marin could keep flowing smoothly through an intersection because cars going left could get out of the way. But that takes space. Assuming that each direction can carry 10.3 cars a minute, that's nearly 21 cars a minute, but spread over three lanes -- two travel, one shared turn. And we're back down to 7 cars per lane minute.( Read more... )

Layman's conclusion: wide roads with little 'turbulence' can get up to 10 cars per lane-minute. More complicated streets are unlikely to get above 8, after accounting for turn lanes. This will have consequences for stuff like "is it physically possible for everyone to drive to work from here?"

In the past, I've estimate the car capacity of city streets with "two second following distance, cut in half for intersections" which yields 15 cars per lane-minute. But what about reality?

Signalized

Today in Philadelphia, around 6 PM, I counted at various intersections. One lane of travel, 30 second light, reliably had 10 cars passing me before running out of light rather than cars. 3 seconds per car.

At much bigger intersection, a near-highway with 6 total lanes of travel, I counted 37 cars in 38 seconds, and 33 cars in 38 seconds. In this case it was the cars that ran out first, I suspect the previous light cutting off supply. 3 lanes in the direction, so once again about 3 seconds per car per lane.

Your big bottlenecks will be where two major streets intersect, each getting green for half the time, so 10 cars in 30 seconds of green is basically 10 cars per minute overall, on average carrying about 15 people per minute.

One articulated bus (120 people) every 8 minutes would double the capacity of a lane, while serving a lot of people who can't or don't want to drive. Or better, turn the lane to a bus lane, keep the same capacity while serving a lot of people etc.

Mostly, it was funny to count 10 cars before the next red light, and think "if one of these was a dinky bus it could be carrying 40 people." Heck, even a little passenger van carries 15 people, one van a minute doubles your capacity.

Stop signs

So much for signalized intersections; what about 4-way stops? I'm not sure; for one thing, my nearby intersections didn't feel like they were at peak traffic. For another, a busy intersection is messy. At first I paid attention to just one lane at a time, and got maybe 6 in a minute, then 7. Later I counted every car going through in all directions (actually the intersection of a two-way two-lane and a one-lane), and got 52 cars in 3 minutes; 3 lanes, so 5.78 cars/lane-minute. But the traffic definitely wasn't fully saturated... Of course, when orthogonal directions are saturated, that slows both down, as do pedestrians. Especially since Philly drivers make rolling stops when they can, so being physically forced to actually stop would slow them down.

I remember that back in Albany, my peak traffic counts were on Marin or San Pablo, about 10.3 cars per lane-minute.

There's a thing I've read about. You take a 4-lane street -- 2 travel lanes in each direction -- and cut it down. 1 travel lane in each direction, plus an alternating central turn lane. Supposedly it's safer, and has about the same throughput -- driver's side turns mean that the inner travel lane tends to get blocked by waiting cars anyway. And of course the conversion gives you space to play with.

I'm pretty sure this has happened to Marin Avenue, north of me, at some point. I had noticed that crossing it was unpleasant even at corners, let alone jaywalking -- it's wide and busy. But when I paid attention, I discovered it only has 1 travel lane each way! But it _is_ wide: each side has parking, a bike lane, and the travel lane, and then there's a rather wide turn lane. So, curb to curb, it's like a 6 lane street.

I'm glad they were able to carve some bike lanes out of the old street, but I wish they had found room for pedestrian bulbs or a median-refuge as well. Or both: bulbs at the corners, say, and a refuge island in the middle of blocks.