Saturday, May 14, 2016

Traffic Models Have Large Errors and Do Not Provide Reliable Projections

How much can we trust the traffic model projections for Lafayette? Read this Journal of the American Planning Assoc. article which finds: "... forecasters generally do a poor job of estimating the demand for transportation infrastructure projects. ... For half of all road projects, the difference between actual and forecasted traffic is more than ±20%. The result is substantial financial risks, which are typically ignored or downplayed by planners and decision makers to the detriment of social and economic welfare." 

Flyvbjerg, B., M. K. Skamris Holm, et al. (2005). "How (In)Accurate Are Demand Forecasts in Public Works Projects? The Case of Transportation." Journal of the American Planning Association 71(2).

During presentations to the community, our Louisiana DOTD has given dire projections of traffic problems that they predict will happen in 2040 if we do not capitulate to building the I-49 Connector through the heart of Lafayette. As pointed out in the JAPA article, there is great uncertainty associated with all such predictions. And, beyond the model error discussed in this professional journal article, other uncertainties complicate the projections for Lafayette:
  1. The model projections being presented to the community assume no tolls will be charged. Will the Connector charge tolls in 2040? We have been told on numerous occasions that tolls are likely the only feasible way to finance this project. Charging a toll will discourage use for local trips, and actually increase traffic on remaining surface city streets. 
  2. If the Connector is not built, what other highway improvements may happen prior to 2040? If the Connector concept is abandoned, it is likely that one of the proposed I-49 bypass routes will be developed. However, if the Connector is completed, it is unlikely Lafayette will see construction of a bypass by 2040.
  3. What improvements in transportation technology will occur over the next 25 years? The model assumes that in 2040 we will still be driving the same basic cars and trucks, carrying the same freight loads on trucks, and using the same traffic management systems that are in use today.
  4. What will Greater Lafayette's population be in 2040 and how will population centers change? The traffic model uses input demographic projections, and quality of the model output is no better than that of the input. We may believe that the trend of population growth south of Lafayette will continue unabated in future decades. However, realities of availability of suitably drained land, and future increases in flooding from the Vermilion and its tributaries may actually drive population growth north of the city. Really, only time will tell.
So, what does a wise investor do in the face of uncertainty? Certainly not construct the massively costly I-49 Lafayette Connector.

This post was based on an earlier Facebook post

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