Unreliable projections

Figure 1. NYMTC projected increases in residents and employment.

The new Scoping Packet (PDF) put out by the Federal Highway Administration for the reconstruction of the Tappan Zee Bridge includes this cute little map, showing "projected" increases in thousands of residents (on top) and jobs (on the bottom). The implication is that these thousands of residents will need to get to the thousands of jobs, and they'll all want to drive, and they'll need the bridge to do it. These numbers all come from the New York Metropolitan Transportation Council, essentially an arm of the New York State Department of Transportation.

Figure 2. NYMTC historical and projected bridge traffic volumes.

A similar argument was made in Figure 2 (from the Alternatives Analysis document that has since been scrubbed from the website), showing the average daily number of vehicles that have driven across the bridge for each year since the bridge was built in the 50s. It's a nice straight line, huh? So extending it, as the dashed line does, seems natural. This projected demand is an important part of the State DOT's argument for replacing the bridge. It's also an enticement for people to buy bonds to finance the bridge replacement: the higher the tolls, the more certain it is that the bondholders will be paid back. But why didn't the FHWA use that chart in the most recent Scoping Packet? Instead they used a chart that only went up to 2010 (Figure 3).

Why did they do that? Well, you might notice that the data for the past decade doesn't quite match projections. In fact, if we compare the actual volumes to the projections, as I do in Figure 4, we find that the projections haven't been very reliable.

Figure 3. NYMTC historical bridge traffic volumes.

The bridge traffic volume has actually been pretty stable over the past decade, in contrast to the dramatic increases that were predicted. There are two likely reasons for this. One is that the high gas prices in 2006-2007 discouraged people from driving, and the unemployment from the resulting recession took away the reason to drive from many west-of-Hudson residents.


However, the rate of increase in traffic volume was low even in the beginning of the decade. The other potential explanation is that the congestion on the bridge, and the high rate of crashes caused by the State squeezing an extra lane in, is a turn-off. Rather than deal with this, people will choose to get jobs elsewhere, or not even move to Rockland.

Figure 4. NYMTC historical (indicated in green from 2002-2010) and projected bridge traffic volumes, including recent totals.

If the volume of traffic is self-limiting (and come on, really, why wouldn't it be?) then there is no urgent need to widen the bridge, or even to replace it. If the bridge capacity is reduced, people will either stop moving to Rockland, Orange and Bergen counties, or they'll move to someplace where they can catch the train to Hoboken.

Even if the new bridge is built, most scenarios call for the tolls to be doubled or even tripled, and many call for them to be indexed to inflation. That would serve to discourage use of the bridge. Furthermore, there is a chance that higher energy prices would limit driving. This could mean that the tolls never bring in enough revenue to pay off the bonds.