On the I-85 Bridge Collapse in Atlanta

A bridge of I-85 over Piedmont Road in Atlanta collapsed yesterday (March 30, 2017) after a fire broke-out underneath (map). Fortunately no one died, as the Bridge was closed by the fire before the collapse. As a former Atlantan, I drove that route many times, especially while working in Norcross at Hayes, and living on campus at Georgia Tech. Screen Shot 2017-03-30 at 19.27.28

The proximate cause for the collapse was the fire. There will undoubtedly be investigations about the causes of the fire, why the fire was severe enough (hot enough) to melt components of the bridge, why whatever was under the bridge was under the bridge, why the bridge collapsed, and so on. As far as I can tell, according to this Transportation for America map, the bridge was not considered structurally deficient or fracture critical. This could very well turn out to be a case of “don’t store hazardous chemicals under bridges” rather than about the sad state of American infrastructure, which is nevertheless sad, but time will reveal the outcome.

Georgia DOT closed the highways (I-85 and parallel Buford Highway connector (old I-85)) soon after the fire, and eventually evacuated all the people trapped on the section of the road upstream of the bridge but downstream of the closure.

The road remains closed today and for the indefinite future. If I-10 in California is the model, it can be probably be reopened in about two months, though perhaps it is more complicated and will take longer. The I-35W Bridge took 13 months to  replace and reopen, but that was a much longer span and was over a major river. (Previous Transportist posts on I-35W)

The first day after the collapse is exceptional, as many people work from home, and traffic is likely to be lighter. Subsequent days, as people try to resume their normal activities will be more complicated, as travelers try to find a new equilibrium (daily variations in traffic are at the same level as before the collapse). We saw this take about 6 weeks in the I-35W case.

When I-35W collapsed, we undertook a number of studies to understand its traffic consequences. A list of relevant papers is below. Preprints are all available for free at the University of Minnesota Digital Conservancy, see links.

In some ways, this will be worse for Atlanta’s traffic than the I-35W bridge collapse was for Minneapolis. Minneapolis has a much more gridlike network, while Atlanta is a ring-radial, with a limited number of ring-routes (mainly I-285, the Perimeter).

Minneapolis had a natural alternative (Mn280 – I-94) which while a small bit longer, and with fewer lanes, and already congested, worked well enough. On the positive side, since the I-35W Bridge was closed, the merge between I-94 and I-35W was simplified. I-94 was restriped to insert an additional lane.

In the Atlanta case, while the merge at I-75 and I-85 will now be simpler for travelers southbound in the morning from Cobb County, travelers from the Northeast who would have taken I-85 will undoubtedly be inconvenienced. Atlanta has MARTA, which serves as a convenient parallel route to those going to destinations near stations (which includes Georgia Tech and downtown), and will undoubtedly see a huge surge in passengers once travelers resume going to work in regular numbers.

Depending on whether the Buford Highway Connector reopens before I-85, will also shape the traffic recovery process. It is the natural bypass, but it remains closed as of this writing, and I am unclear if it was structurally affected by the fire.

Atlanta long ago had plans for parallel freeways through the city, (map) which were cancelled in the era of freeway rebellions. While cancelling the roads was probably good policy at the time, as more roads do induce more traffic with all of the negative externalities that brings (though of course roads tend to increase consumer surplus as well), such cancellation does make the highway network more vulnerable to catastrophic failure. So many vehicles use individual links like I-85, and especially the Downtown I-75/I-85 connector, that when they are closed for whatever reason (collapse or even construction), there is no slack in the system to absorb them. Risk severity is an important issue, some links are more critical than others in maintaining the accessibility of the system.

Longer-term, I expect the I-485/GA-400 extension that was cancelled in the 1970s will again become a topic of conversation. All the cool cities (Seattle, Sydney) are building Big-Dig like tunnels, at extraordinary cost, but with far less social impact than surface highways. This would not surprise me as an outcome here. While neighborhood opposition was enough to kill it when no one else cared passionately, the issue will again be front-and-center.

I expect MARTA will get more support as well, as people see the need for some network redundancy (more than one path), in this case, technological redundancy (more than one mode). Redundancy and variety begets stability. What if this event had happened along I-75 without MARTA? What if a tanker exploded at a major interchange?

In the shorter term, the surface streets will accommodate much more traffic into Downtown. The network in the region of the collapse is not gridlike, unlike downtown, so this will be a natural pressure point. While some things can be done with traffic signals to prioritize flows in different directions, in the end, the capacity at intersections is finite.

Demand management strategies will also be tried, carpooling will be encouraged, and employers may be more willing to adjust start and end times of work to accommodate traffic.

While comparisons are natural with snowmaggedon, this is a very different type of case, a highly localized, but longer term failure of a link, rather than a systemic but shorter term failure of the network as a whole.

Selected Relevant Papers