Unlike bridges, transportation networks are seldom “fracture critical”. While the Interstate Highway System did what it could to sever local streets and channelize traffic onto fewer, larger, limited access link to achieve economies of scale and higher speeds and throughputs and the expense of redundancy, there was enough remaining redundancy to ensure that this one bridge collapse would not have devastating transportation implications. Had the I-94 bridge collapsed instead, I am sure the consequences would have been much worse, as the alternative paths are not as convenient. But again, the Twin Cities would have muddled through.

Several lessons can be drawn. If there is redundancy, maintaining road operations during construction may not be necessary, and may needlessly delay construction while exploding its cost. There have been projects, in Minnesota notably Mn36, where closing the road entirely to speed construction has been successful. Rebuilding a bridge while keeping it open is like doing brain surgery on one-self. In principle it is possible, but why? The length of bridge construction in other examples (e.g. the Oakland Bay Bridge, admittedly a somewhat more complicated project)
versus the less than one year from design to opening that this bridge took is instructive.

Actual effects were much less than forecast. People are quite adaptable in their travel patterns, especially if there are alternatives routes and destinations.

As a consequence of the Bridge collapse, many other bridges were repaired more quickly than they otherwise would have been. A list of recent Bridge closures in the region below is illustrative.

1. August 1, 2007: I-35W
2. January 2008 Hastings Bridge
3. March 20, 2008 St. Cloud Bridge
4. March 26, 2008 University of Minnesota Pedestrian Bridge
5. April 25, 2008 Lowry Avenue
6. May 6, 2008 Blatnik Bridge (I-535) in Duluth
7. June 4, 2008 MnDOT barricades Hwy. 43 bridge over Mississippi River at Winona

Three Successes

• Emergency Response
• Immediate Traffic Restoration
• Quick Completion of Bridge with Design/Build

Three Failures

• Bridge Collapse
• Removal of Successful, Low Cost Restoration Measures
• Overpaying for Bridge

Overall, Americans seem good at short-term tactics but poor at longer term strategy. This needs to be rectified. While there were positive outcomes, including the passage of the gas tax, and increased attention to maintenance, there may have been some over-reaction in terms of replacement of bridges.

While there was some accountability, some key personnel remained in place. Firings and resignations need to be used more often in the aftermath of events like this, some people should assume responsibility.

We need to address the question of the appropriate role of politics in infrastructure decisions. We don’t expect politicians to make recommendations about which electric power cables are replaced, why are they so involved in maintaining existing roads. (The construction of new roads is on the other hand, more obviously political in nature). Should Mn/DOT and other similar agencies move toward a public utility model to de-politicize?

We are not good at dealing with low probability, high consequence decisions. We are not good at assessing the value of either.

Unless something is done, failure will be more frequent, the Interstate system is aging and nearing the end of useful life for many components.

In The Transportation Experience, we identify a set of strategies for maturity:

• Abandonment
• Cash cow – using resources for something else
• Maintenance and Rehabilitation
• Replacement

Abandonment of the Interstate system as a whole seems premature, and while a handful of selected urban links have been closed, these are very much the exception. The Cash cow strategy is in fact what has been employed for the past several decades, as the gains from the built infrastructure exceeded by far the amount that was reinvested in it. This is of course economically correct, infrastructure should be fully exploited to its capacity. But this needs to be coupled with appropriate levels of maintenance and rehabilitation, and ultimately planned replacement, otherwise the goose that lays the golden egg will die.

The “Weak Bridge” sign that one sees all over England is not terribly reassuring, and indicates an unwillingness to recapitalize the network. In the US context, there should be more money for transportation maintenance and rehabilitation, we are eroding our physical capital. If there isn’t … we should spend our money more carefully, taking care of the existing systems and users first, and engaging in graceful abandonments as necessary, but not building new infrastructure which will require long-term maintenance without any means for doing so. The mantra Fix it first has been suggested for this strategy, and I agree. Unless something is done, the problem of decaying infrastructure will only get worse (the physical world being subject to entropy and all), bridges do not repair themselves.

Other Parts in Series: Part 1 – Introduction, Part 2 – Structure, Part 3 – Communication,Part 4 – Politics, Part 5 – Economics, Part 6 – Traffic, Part 7 – Replacement, Part 8 – Policy Implications

The replacement bridge cost $251 million, funded almost entirely by federal government. We can debate whether the federal government should have paid for it (it was originally built with 90 percent federal contributions, 10 percent state, but matches recently are much more balanced), since most traffic using the Bridge both originated in and is destined for Minnesota. With Minnesota Congressman James Oberstar then Chair of the House Transportation Committee, there was plenty of political support behind this.

The replacement bridge was hurried, completed by September 2008, several months ahead of the original schedule. This is good, a lack of a bridge costs the economy. (Xie and Levinson estimated between $127,000 and $170,000 per day, MnDOT estimated $400,000 per day). The contractor received $200,000 per day bonus for early completion. So perhaps in an economic sense, too much was paid to complete it a few months early.

Rebuilding a collapsed bridge is of course a crisis, but it is also an opportunity to do something interesting. Rushing designs may mean that ideas are missed. What was built is a functional bridge, and there are state-of-the-art real-time structural health monitoring systems installed, so I have no fear driving over it. It was also ensured that the Bridge would be compatible with any future light-rail transit lines, though none are planned for this bridge (and how they would transition from the center of the Bridge to any reasonable destination is extremely unclear). But could more have been done?

The snow removal and icing problem was not considered. Minnesota is famous for its winters. The previous I-35W bridge had installed a de-icing system (in response to earlier crashes), which had been speculated to be responsible for corrosion of the structure. While the NTSB did not find that, de-icing chemicals do have environmental consequences.

A solution not considered was air rights. A bridge over the Mississippi is expensive. But imagine having a 2 or 3 story office building hanging from below, and/or built above the highway. The views from the River are fantastic. It will not impair other views of the river especially much, and would generate a significant amount of revenue to pay for reconstruction. One example would be the historic London Bridge, which had houses and stores along the side, encroaching on travelways. There are better ways to combine transportation arteries with development opportunities, and creative design can show the way.

Why did we not build a habitable structure above the road deck, so that offices, residences, hotels, etc. could share the view? It would have increased the profile of the Bridge by two stories, but created an enormous amount of leasable space, leases which could have paid for themselves and the Bridge. Obviously it would increase the initial construction cost, and perhaps time, but that would be amply repaid over the long-term. That structure would further have shielded the roadway from ice and snow, reducing road snow clearance costs and crashes.

The bridge opened in the early darkness of September 18, 2008. A parade of first responders, and then a bulge of traffic all hoping to be the first (and none succeeding) went across. Soon the Bridge was attracting 120,000 vehicles per day, measurably off the pre-collapse levels.

Other Parts in Series: Part 1 – Introduction, Part 2 – Structure, Part 3 – Communication, Part 4 – Politics, Part 5 – Economics, Part 6 – Traffic, Part 7 – Replacement, Part 8 – Policy Implications

Travel behavior changes after network disruption as well as after the replacement of disrupted links are not well-understood. The first table shows the number of river crossing trips by type of facility before and after the collapse and the reopening.

We discover 46,000 lost trips daily after the collapse (nearly a third of what the I-35W Bridge had carried), and 20,000 found trips after the new bridge opened. Those lost trips may not have been made, or more likely, found different destinations not requiring a river crossing. This provides additional evidence to the phenomenon of induced demand.

Gains from the Bridge for three peak periods re-estimated with accurate (observed) travel times (but fixed and not-observed OD matrix), shown in the second Table, were on the order of $70,000 per day (somewhat below our initial low all-day estimates ($127,000), far below Mn/DOT’s, and also below the contractor early completion bonus). Much of that gain is lost once the I-94 bridge lane disappears, as consistent with the original results. $42,000 annualizes to about $15 million in benefit, for a $250 million dollar bridge (which pays off in about 23 years at 3% interest). The I-94 lane restriping paid off in a matter of a month.

Other Parts in Series: Part 1 – Introduction, Part 2 – Structure, Part 3 – Communication, Part 4 – Politics, Part 5 – Economics, Part 6 – Traffic, Part 7 – Replacement, Part 8 – Policy Implications