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[CoreySamson]

I recently gave some thought to the practicality of double decker freeways, and whether they should be brought back. In 1989, the Loma Prieta earthquake destroyed the Cypress Street Viaduct, killing many and with its collapse new double decker freeways in the US have never seen the light of day due to safety reasons. I was thinking, surely these freeways wouldn't be good for places like California, where earthquakes are common, but could they be used in other parts of the country?

Chicago, for instance, has a ton of surface streets that are multilevel, and there are some major river crossings that use double decker cantilever designs, but a lot of these double decker designs seem to be dated. I think that in places that don't have to deal with earthquakes or tornadoes where space is at a premium, new innovative multilevel designs could save space and improve traffic capacity. I thought of the following plan for the Pierce Elevated in Houston, combining aspects of a sunken freeway and a double decker highway:

Upper Deck (northbound, six lanes):               _ _ _ _ _ _
Surface Level (cross streets, car parks, etc.)_____________
Lower Level (southbound, six lanes):            \ _ _ _ _ _ _ /

In case of flooding, the lower level could hold water and the top deck could temporarily be converted into a two-way road. The only downfall to this plan that I see would be the cost.

Does anyone know why double decker highways aren't more common than they are? To me it seems they seem like a great way to save space, but is the cost of them just too great?

[SteveG1988]

I recently gave some thought to the practicality of double decker freeways, and whether they should be brought back. In 1989, the Loma Prieta earthquake destroyed the Cypress Street Viaduct, killing many and with its collapse new double decker freeways in the US have never seen the light of day due to safety reasons. I was thinking, surely these freeways wouldn't be good for places like California, where earthquakes are common, but could they be used in other parts of the country?

Chicago, for instance, has a ton of surface streets that are multilevel, and there are some major river crossings that use double decker cantilever designs, but a lot of these double decker designs seem to be dated. I think that in places that don't have to deal with earthquakes or tornadoes where space is at a premium, new innovative multilevel designs could save space and improve traffic capacity. I thought of the following plan for the Pierce Elevated in Houston, combining aspects of a sunken freeway and a double decker highway:

Upper Deck (northbound, six lanes):               _ _ _ _ _ _
Surface Level (cross streets, car parks, etc.)_____________
Lower Level (southbound, six lanes):            \ _ _ _ _ _ _ /

In case of flooding, the lower level could hold water and the top deck could temporarily be converted into a two-way road. The only downfall to this plan that I see would be the cost.

Does anyone know why double decker highways aren't more common than they are? To me it seems they seem like a great way to save space, but is the cost of them just too great?

Cost is high, post 1989 you have to take a lot more into consideration in the design stage, they are kind of ugly, block out the sky from the street level, etc. Tunnels are more common now due to that.

[jeffandnicole]

Cost is sky high. I'd imagine it's about 10 times the cost of a regular roadbed. Because of the need for supports, the room saved isn't all that much. And they need to be inspected every 1-2 years, depending on the age of the structure.

[Hwy 61 Revisited]

I could imagine a double-decker suspension bridge for a freeway. Not so sure about a double-decker all around.

[Revive 755]

There are also issues with winter weather and repairs/inspections being more of a problem.  It can be harder to stage work on the upper deck, and both directions may have to be closed for significant work.  With a side by side arrangement traffic could be diverted to the other side while one side is being worked on, albeit at reduced capacity.

[D-Dey65]

The only place I can think of where a double-decker freeway would be useful is at the site of the Oakdale Merge, and that's only with the lower level as the service roads connecting the two segments of Montauk Highway.

[jakeroot]

From an urban design point of view: they're just not pretty at all. They create a wall (physical and psychological) no matter where they're placed.

The best freeways are those that people can use like any other, but which don't screw with the land around which they run. Tunnels are the greatest thing ever for this exact reason. Expensive as they are, though. Probably why they're still not common.

[Hwy 61 Revisited]

Again, double-decker suspension bridges (over rivers and such) are totally cool.

[oscar]

From an urban design point of view: they're just not pretty at all. They create a wall (physical and psychological) no matter where they're placed.

Think the late Embarcadero Freeway (I-480, then CA 480), which created exactly that effect on San Francisco's waterfront until it was demolished after the Loma Prieta earthquake.

I'm not sure other double-decker freeways in the Bay Area were as bad. The double-deck Cypress viaduct, which collapsed in the Loma Prieta quake, was despised for splitting the community it sliced through. But I think a single-level freeway would've been about as bad. When it came time to replace the Cypress, the community wanted the replacement freeway to bypass it altogether. That is what happened, with a ground-level boulevard (Mandela Parkway) built in the old ROW. 

[jakeroot]

^^^
Not to mention the local-to-me Alaskan Way Viaduct, which was demolished not just because it was physically failing, but also because no one really liked it. It was functional, and that was it. And it wasn't even that functional late in life given how often it was shut down for maintenance and inspections.

Single-level freeways seem much more tolerated. They're arguably worse to walk beneath since they last longer, but at least they don't tower over neighborhoods.

[SteveG1988]

Cost is sky high. I'd imagine it's about 10 times the cost of a regular roadbed. Because of the need for supports, the room saved isn't all that much. And they need to be inspected every 1-2 years, depending on the age of the structure.

GOod pun.

[ilpt4u]

The only double-decker freeway (that is not a river crossing bridge) I have experience with is I-64/US 40 in St Louis, with its 2 short double-decker segments. I don’t think anything is inherently wrong with the design. But there is probably a reason the St Louis double-decker is 2 shorts segments instead of one long double-decker from the PSB to Market & Vandeventer

[Bruce]

From what I understand, it depends on the design of the columns. There's plenty of examples of double-decker structures in other cities (including those vulnerable to earthquakes) around the world, but they don't use different designs.

In Seattle alone, there's also the double-decker portions of I-5 (the express lanes between Seneca and Mercer) and I-90 (the section west of I-5) that have different designs from different eras. The former is potentially vulnerable along with much of the other 1960s sections due to their weaker columns.

(Note I am not an engineer and can only talk in broad terms)

[michravera]

I recently gave some thought to the practicality of double decker freeways, and whether they should be brought back. In 1989, the Loma Prieta earthquake destroyed the Cypress Street Viaduct, killing many and with its collapse new double decker freeways in the US have never seen the light of day due to safety reasons. I was thinking, surely these freeways wouldn't be good for places like California, where earthquakes are common, but could they be used in other parts of the country?

Chicago, for instance, has a ton of surface streets that are multilevel, and there are some major river crossings that use double decker cantilever designs, but a lot of these double decker designs seem to be dated. I think that in places that don't have to deal with earthquakes or tornadoes where space is at a premium, new innovative multilevel designs could save space and improve traffic capacity. I thought of the following plan for the Pierce Elevated in Houston, combining aspects of a sunken freeway and a double decker highway:

Upper Deck (northbound, six lanes):               _ _ _ _ _ _
Surface Level (cross streets, car parks, etc.)_____________
Lower Level (southbound, six lanes):            \ _ _ _ _ _ _ /

In case of flooding, the lower level could hold water and the top deck could temporarily be converted into a two-way road. The only downfall to this plan that I see would be the cost.

Does anyone know why double decker highways aren't more common than they are? To me it seems they seem like a great way to save space, but is the cost of them just too great?

I had a boss who lived in Dallas while I lived in California. He had a chance to move either to Atlanta or San Jose. He said that he would never live in California because he was afraid of earthquakes. While I was on the plane back to San Jose after that trip, an 'earthquake' hit Atlanta. I think that he tried to work out staying in Dallas (and then an earthquake hit there).

[jakeroot]

In Seattle alone, there's also the double-decker portions of I-5 (the express lanes between Seneca and Mercer) and I-90 (the section west of I-5) that have different designs from different eras. The former is potentially vulnerable along with much of the other 1960s sections due to their weaker columns.

What do you think the completion of I-90 did in the 1980s? Because I know that portions of that double deck section of I-90 were largely built in the 1960s, with an extension in the 1980s:

https://twitter.com/wastatearchives/status/1245054071603523585

[cbeach40]

Ontario has considered adding a second deck to Highway 403 where it runs along the Niagara Escarpment in Hamilton.
https://goo.gl/maps/iJtzAYYSefkJ4n9j7
Given the environmental sensitivity of the area it would allow them to expand the highway without expanding the footprint. That alternative hasn't really gained much traction and may be passed on in favour of a new route to the north.

[jakeroot]

Ontario has considered adding a second deck to Highway 403 where it runs along the Niagara Escarpment in Hamilton.
https://goo.gl/maps/iJtzAYYSefkJ4n9j7
Given the environmental sensitivity of the area it would allow them to expand the highway without expanding the footprint. That alternative hasn't really gained much traction and may be passed on in favour of a new route to the north.

MTO needs to hire some engineers from BC! They'd make that tiny central reservation into a new eastbound lane.

[interstate73]

Ontario has considered adding a second deck to Highway 403 where it runs along the Niagara Escarpment in Hamilton.
https://goo.gl/maps/iJtzAYYSefkJ4n9j7
Given the environmental sensitivity of the area it would allow them to expand the highway without expanding the footprint. That alternative hasn't really gained much traction and may be passed on in favour of a new route to the north.

Has the same been considered for the 401 between the 427 and the 409? The time's I've been to Toronto I've loved the dual carriageway setup but it always feels like the congestion from that bottleneck negates the traffic flow benefits of such an expansive highway setup!

[Hwy 61 Revisited]

[fictional]
I've been considering reconstructing the Hill-to-Hill Bridge in Bethlehem as a double-decker suspension bridge in order to save space for a potential expansion down to I-78.
[/fictional]