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Comment by h_tbob | Hacker News Reader

As somebody who lived in Dallas and drove on the high five with some frequency, I had actually a lot of questions about the design of it. If somebody who understands this could explain I’d be happy.

For one, a lot of the overpasses had two lanes. But then they merged down to one right before you got on the highway…

What’s the point in having two lanes on the overpass that merge just before you get onto the highway? There was always slow traffic because of this.

If it was me, I would always make it so that you lose equal number of lanes To the outgoing overpasses as you will get in incoming overpasses to prevent inefficient lane changes. So if you have six lanes, you get 4 branch off, 2 for each direction on the other highway and keep 2. Then you get the four back from the intersection. No lane change.

But the way they have it, there’s so much merging it , which is harder to drive, and I’m not sure it’s more efficient.

Any thoughts?

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There's a cities skylines YouTuber who agrees with you. He calls it lane math. "2 come off, 2 go on,..." I'm sure there's some reason the engineers do it irl, but it is funny that going with your approach does fix things in game.

I have no answer, but this got me thinking: is the overpass's road a similar physical width as the highway?

I've been on some bridges that have two lanes with a very narrow shoulder that then then merge into a single lane (with wider shoulders) at the end of the bridge, with the asphalt width not changing too much. I've always assumed that was so traffic could continue flowing on the bridge should one lane become blocked for some reason. Off the bridge a car can pull off the road if there's a problem to let traffic continue passing. You can't really do that so much on a bridge.

But I'm not a traffic/road engineer so that's just speculation on my part.

My guess is the two lanes merging into one are only getting one lane. That is, only one lane is added after the merge, and both overpass lanes are merging into the one new lane.

But if there are indeed being two lanes added due to the merge, agree that doesn’t make much sense.

If the former, also agree that they should’ve added two lanes instead of one. But that decision could’ve come down to physical limitations, cost, or something else.

I completely agree. The ramp from 75 south to 635 west squeezed two lanes into one before it merged with 635, and it was always an enormous bottleneck when there was any traffic at all. But the last time I drove on that ramp was 5-6 years ago, maybe it's better now.

I don't have experience with Texas but there are plenty of interchanges as you describe in California. Sometimes you even see momentarily more lanes just before and after an at-grade intersection.

While more permanent lanes seems more desirable, for lack of space or money or whatever, this compromise at least increases the capacity of the ramp to absorb the queue before it starts to back up into through traffic.

Of course, in practice the capacity is often not enough.

I wonder if it's more about having room for emergency vehicles or for people to pull over if there's an accident without totally stopping traffic.

Depends on setup, but if the alternative is to merge before splitting off, that can slow down the main highway. Get the two lanes separated first, then merge.

Around here, two lanes split, then they split into two north and two south, and then they merge again before rejoining the cross highway.

I haven't seen the road in question, but I wonder if it has to do with expected travel speed. Two lanes at low speed converting to one at higher speed. Of course following distance impacts this, but isn't actually 1:1 with speed

I always figured it was so that the bridge acts as a storage buffer for backed up traffic so you don't get the ramp to 635 eastbound so backed up that there is stopped traffic on 75 southbound.

I mean you still end up with that at rush hour but I think that's what they are trying to do. There is no number or lanes they could add that would prevent it from being a parking lot twice a day.

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