Comment by toast0
5 hours ago
The slow lane and passing lane dichotomy makes sense in a rural highway with two lanes in your direction.
It makes less sense in an urban environment with 5 or more lanes in your direction. Vehicles will be traveling at varying speeds in all lanes, ideally with a monotonic gradient, but it just doesn't happen, and it's unlikely to.
In California, large trucks generally have a lower speed limit (however many trucks are not speed governed and do exceed the truck limit and sometimes the car limit) and lane restrictions on large highways. Waymo may do well if it tends toward staying in the lanes where trucks are allowed as those tend to flow closer to posted car speed limits. But sometimes there's left exits, and sometimes traffic flow is really poor on many right lanes because of upcoming exits. And during commute time, I think the HOV lane would be preferred; taxis are generally eligible for the HOV lane even when only the driver is present, but I don't know about self-driving with a single or no occupant.
Isn’t the situation you’re describing where speeds vary due to queues going to be in heavy traffic where cars aren’t getting close to the speed limit anyway?
(also it’s kind of amazing that 5 parallel lanes is considered normal in the US… I think the most I’ve ever personally seen in the UK is 4 and that’s only on very major routes, and we don’t have any exits on the wrong side of the motorway)
5 each way is decently big imo but it really depends on the area. A freeway near a really large metro that's particular car-dependent can easily go higher.
> Isn’t the situation you’re describing where speeds vary due to queues going to be in heavy traffic where cars aren’t getting close to the speed limit anyway?
Not necessarily. I've seen things like the left two lanes at free flow (speed limit or above) and the right two lanes at full congestion (~ 10 mph), and the middle lane(s) somewhere in between. But then you also have sometimes where the left lane is only doing 60 for some reason, but the next two lanes are at or above the speed limits. It's a complex system.
Wrong side exits for interchanges between highways are common, depending on site details and relative flows. When there's congestion on a left exit, you then get situations where the right lanes are flowing faster than the left lanes (sometimes much faster). I don't think interchanges as left exits are necessarily awful.
Wrong side exits to surface streets have been discouraged for new construction for quite some time, but there's a fair number of "legacy exits" in some areas. They're not so bad when there's only two lanes in your direction; but when there's been highway expansion, it can get pretty hard to use. And inevitably rebuilding to current standards would causes a lot of confusion and delay, it's postponed. My exemplar of the worst left exits, the Milwaukee Zoo Interchange, was rebuilt in 2012-2022 and I can't find pictures of what it was before, but you had a sizable interchange with right and left exits to other highways, combined with several surface street exits and entrances on both sides, and I think two through lanes. It was a mess.