How do highway engineers choose between over/underpasses for interchanges?

[kernals12]

Intuitively it would seem like overpasses would be cheaper because it negates the need for excavation, but here in Maricopa County's East Valley, freeways almost always pass under cross streets. Did Mesa, Gilbert, Chandler, and Tempe successfully extort ADOT into paying more to build freeways just for aesthetic benefits or am I wrong and having the highway go under cross streets is cheaper?

[pderocco]

There's a commonly used construction technique where the overpass is built on the ground, and then undercut to make room for the freeway. Meanwhile, the future freeway traffic uses what eventually become the diamond ramps.

You can see a lot of this in the Google Earth historic imagery for the Las Vegas beltway.

[kernals12]

There's a commonly used construction technique where the overpass is built on the ground, and then undercut to make room for the freeway. Meanwhile, the future freeway traffic uses what eventually become the diamond ramps.

You can see a lot of this in the Google Earth historic imagery for the Las Vegas beltway.

And they're doing that now here in AZ on SR 24.

[Beltway]

Intuitively it would seem like overpasses would be cheaper because it negates the need for excavation, but here in Maricopa County's East Valley, freeways almost always pass under cross streets. Did Mesa, Gilbert, Chandler, and Tempe successfully extort ADOT into paying more to build freeways just for aesthetic benefits or am I wrong and having the highway go under cross streets is cheaper?

Mainly topography. The simplest is to have the local road overpass the freeway.

When the topography makes that difficult, that is when a better design is to go under the freeway.

Regarding urban freeways with lots of closely spaced local road crossings, they can choose between passing over all the streets or a depressed highway passing under the streets.

[pderocco]

Intuitively it would seem like overpasses would be cheaper because it negates the need for excavation, but here in Maricopa County's East Valley, freeways almost always pass under cross streets. Did Mesa, Gilbert, Chandler, and Tempe successfully extort ADOT into paying more to build freeways just for aesthetic benefits or am I wrong and having the highway go under cross streets is cheaper?

Mainly topography. The simplest is to have the local road overpass the freeway.

When the topography makes that difficult, that is when a better design is to go under the freeway.

Regarding urban freeways with lots of closely spaced local road crossings, they can choose between passing over all the streets or a depressed highway passing under the streets.

Since most of the Las Vegas beltway, or at least the newer parts, are below grade, I think it may also have to do with sound control. Having the road at a lower level than the sound walls creates a sound shadow that projects upward even more, so the adjacent homes hear it even less, at least for the high frequencies.

[kernals12]

Regarding urban freeways with lots of closely spaced local road crossings, they can choose between passing over all the streets or a depressed highway passing under the streets.

I see. And I suppose that in that scenario, bringing in a whole lot of dirt for embankments is just as expensive as removing dirt for a depressed roadway?

[vdeane]

There's a commonly used construction technique where the overpass is built on the ground, and then undercut to make room for the freeway. Meanwhile, the future freeway traffic uses what eventually become the diamond ramps.

You can see a lot of this in the Google Earth historic imagery for the Las Vegas beltway.

How does that work when the road isn't initially constructed as an overpass?  They'd still have to build the bridge; they can't just excavate under asphalt.

[sprjus4]

There's a commonly used construction technique where the overpass is built on the ground, and then undercut to make room for the freeway. Meanwhile, the future freeway traffic uses what eventually become the diamond ramps.

You can see a lot of this in the Google Earth historic imagery for the Las Vegas beltway.

How does that work when the road isn't initially constructed as an overpass?  They'd still have to build the bridge; they can't just excavate under asphalt.

Construct an overpass with very little clearance directly next to the existing road, shift traffic onto the new bridge, demolish the existing road, then excavate out underneath the new overpass and old road to create the proper 16 ft clearance needed.

[Beltway]

Regarding urban freeways with lots of closely spaced local road crossings, they can choose between passing over all the streets or a depressed highway passing under the streets.

I see. And I suppose that in that scenario, bringing in a whole lot of dirt for embankments is just as expensive as removing dirt for a depressed roadway?

Generally in the same realm for earthwork. The depressed highway would require drainage outfalls to drain rainwaters away, and possibly electric pumps as well.

The elevated highway could also be built on a long bridge that passes over the local streets.

Since most of the Las Vegas beltway, or at least the newer parts, are below grade, I think it may also have to do with sound control. Having the road at a lower level than the sound walls creates a sound shadow that projects upward even more, so the adjacent homes hear it even less, at least for the high frequencies.

Depressed highways definitely are less noisy to the adjacent development. One of the commonly cited benefits.

[kphoger]

Someone really needs to use this thread title as the daily question in the "worst possible answer" thread...

[fillup420]

my initial thought is that its cheaper to overpass the freeway, since that usually means just one bridge instead of two, and then the freeway is on the ground level, meaning any bridge issues would close the intersecting road instead of the freeway itself.

[kernals12]

The elevated highway could also be built on a long bridge that passes over the local streets.

That's way more expensive

[Beltway]

The elevated highway could also be built on a long bridge that passes over the local streets.

That's way more expensive

Than building it on embankment fill? Not necessarily. That still requires mainline bridges over each local road. All that soil fill material has to be excavated from borrow pits, and borrow excavation is typically twice the cost per cubic yard as compared to regular excavation (from the right-of-way).

[pderocco]

There's a commonly used construction technique where the overpass is built on the ground, and then undercut to make room for the freeway. Meanwhile, the future freeway traffic uses what eventually become the diamond ramps.

You can see a lot of this in the Google Earth historic imagery for the Las Vegas beltway.

How does that work when the road isn't initially constructed as an overpass?  They'd still have to build the bridge; they can't just excavate under asphalt.

Yes, they do build a bridge, but on or close to ground level. Perhaps it's just easier to do that than to build it 20 feet in the air. I don't know why, but I've seen it done in several places, including when CA-210 was extended from I-15 to I-215, and I'd occasionally drive along next to the construction zone to see their progress.