City street lane capacity 3: left turn lanes

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?"