On my way to school this morning I was traveling by some contruction that is widening the highway to allow for better traffic flow. With more people come more cars.
What are the benefits and/or problems if widening was replaced with stacking. Whereas all commuter traffic goes on the bottom, and through traffic goes on top. Or just will-nilly to both, go where ever you want.
Making a three lane highway into a six lane highway with no widening seems quite logical to me. Some logistical problems do exist such as diverting traffic for construction etc…etc… but what other problems are there? Cost?
Entire highways do not need to be stacked but maybe parts too and from large cities could be. Am I missing something that is inherently wrong with this idea?
It is done in a lot of places. Austin has had I-35 double-decked past downtown and the University for a long time. San Antonio has recently double-decked I-10 north of downtown and is currently stacking parts of Loop-410. The I-10 project in San Antonio did not involve closures or other disruption significantly different than a widening project would have, and it allowed them to increase lanes in the same footprint without destroying existing businesses nearby. I believe widening a lot of these roads would be prohibitively expensive if they had to acquire all the commercial property alongside.
A lot of people (say, me) feel nervous when driving on the lower deck of double-deck freeways following the collapse of the Cypress Freeway, I-880, in Oakland in October 1989.
An additional problem is that when you elevate the freeway, you increase the area exposed to the noise of the freeway and block views. While I’m sure context-sensitive design can address that to a degree, the fact is most people don’t want a 20-foot high wall of concrete and noise dividing their neighborhoods. Boston is in the process of tearing down its elevated, double-decked freeway through downtown and moving it underground in the Big Dig.
It would cost a lot more, because instead of simply putting a roadway on the ground, you’d have to design a structure to support the roadway. In most cases, except where property values are relatively high, it’s cheaper to acquire more land than it is to build and maintain the structure. You also have to remember that it’s not just the freeway that needs to have a structure; on- and off-ramps will need them too, and overpasses would need to be rebuilt to be twice as high.
In earthquake country, people would be reminded of the 1989 Loma Prieta quake, where a double-decked freeway collapsed, killing dozens.
With no place to pull off the road, breakdowns would cause worse traffic jams, and it would be more difficult to maneuver emergency vehicles. They generally don’t design double-decked structures to have emergency lanes.
People that live near the freeway would complain; it’s bad enough to simply live near a freeway, but sound walls can mitigate the noise. But when you elevate the traffic above the sound walls, there’s not much you can do about it, at least not for a reasonable cost. And nobody wants to step into their backyard and see a freeway looming 30 feet above you.
Diverting traffic during construction is more difficult too. When widening, it’s not as difficult to move traffic around the construction. But for double-decking, there’s no way to go around it. And because of the more complicated construction, it would take a lot longer to build, too.
Anyway, that’s just some of the reasons why double-decking is generally avoided if possible.
In some cases, the on/off ramps are limited on the upper deck to reduce this cost. For example, on I-35 in Austin there is no opportunity to get on or off the upper deck except where it begins and ends. This works well in this case because it’s a relatively short stretch and a significant portion of the traffic is not local.
This is the Cypress structure mentioned by SanibelMan. The Bay Bridge is double-decked as well, and a portion of it collapsed in the same quake. However, this risk is fairly insignificant outside active earthquake zones and often not a deciding factor even when quakes are an issue. People die on non-stacked freeways every day.
Most of the double-decked roadways I’ve seen have multiple lanes and wide shoulders. I’ve never seen a case where the upper deck was completely blocked by an accident though I’m sure it happens. However, given the sound walls and other barriers used on all roads, I think it’s equally likely that a ground-level non-stacked road would be blocked by a breakdown or accident.
Neighborhood impact is certainly an issue. This is what killed the elevated Embarcadero project in San Francisco when residents realized it would completely block the Bay view for a significant part of the city. As always, this has to be balanced against traffic requirements and the expense of acquiring additional width.
The San Antonio I-10 project used a cantilevered design with the supports for the upper deck completely on one side of the ground-level roadway. This allowed them to complete a significant amount of the construction without disrupting the existing road. There were certainly some lane closures as they installed the last parts of the upper decks, but I don’t think it was too much worse than a widening project would have been and I don’t believe the interstate was ever completely closed in either direction.
I think the real solution is reversible lanes. I’ve seen them in Houston. They are closed mid-day, and opened with the flow of the heavy traffic in the morning and evening.
Reversible lanes only work if the dominent traffic flow that varies by time.
Most large cities have a “ring” freeway that goes around the city, and that often has heavy traffic in both directions at rush hour times. So reversible lanes won’t help there. But for in/out freeways, they would be useful.
For a few weeks, at least, until the traffic increases to fill these new lanes. There seems to be a lot of information indicating that the more lanes of freeway you build, the more traffic that will be attracted to it. Maybe the “real solution” is a working transit system?
P.S. About the collapsed double-deck freeways in California, I remember reading that various parts of those were built by different contractors, and that the ones that collapsed were all built by the same contractor. And ones built by other contractors, right next to them, stood without collapsing. I remember seeing photos that seemed to confirm this – sections where it all collapsed, then all of a sudden switching over to sections where nothing collapsed.
I do remember that it was suggested that this one contractor may have added a bit more sand and skimped on the cement in the sections he built, and that this is why his sections collapsed while other contractors stood. But I don’t recall hearing about any investigation or any results on this. Anybody from out in CA have more info on this?
This is a real issue, especially when you add more lanes to a already main road. A good alternative is to build (or build up) alternate routes. But in general few will take any alternates until the main road goes over 100% capacity (actually slowes down at around 80%, and the only ones who will take alternates will be people for who the alternate is there main route).
Mass transit, unlike roads, gets faster as more people use it (to a point, and this is mainly sceduling, light use might get 1 train / hr, heavy subway usage might have a train every 3 minutes). So the more jammed the roads are, the more people who will use mass transit. Free up the roads and you could desert the mass transit system.
Well, not L.A., New York, right? I’m trying not to be snippy here, but what percentage of cities have a ring, that also have an MLB or NFL team (just as a definition of “large city”)?
Never heard that before. Sections that failed were by and large over man-made fill, while sections that remained upright were built on firmer ground. You can find out a bit more about it at the USGS. Some photos are also available at Ladder 54’s webpage.
a ring doesn’t have to be defined as such, any road that gets heavly commuted both ways at the same time would qualify. NYC metro area has many ring roads. Usually a 3 digit interstate # if started w/ an odd number is a ring, even is a spoke.
I-696 in Michigan could benefit from reversible lanes. In the morning, westbound traffic is far heavier than eastbound traffic, and in the evening the eastbound traffic is heavier. This is due mostly to people who live in Detroit or it’s northern suburbs, but who work in plants in the western suburbs. By a stroke of good luck, I head east in the morning and west in the evening
