Yesterday afternoon, Providence Journal reporter Katherine Gregg tweeted a table from a new CDM Smith report that the RI Dept. of Transportation (RIDOT) just released as part of its campaign for a huge toll-and-borrow plan.  According to the table, out-of-state truckers account for 55.3% of truck traffic in Rhode Island, and after accounting for a multi-trip discount applied to tolls in the state, they’d account for 60.2% of the tolls collected.

In today’s related article, Gregg quotes Governor Gina Raimondo as saying that her plan shifts the infrastructure burden toward out-of-state truckers, who are “abusing” Rhode Island’s roads.  But a very important detail is completely left out of the CDM Smith report: diversion.  RIDOT’s original projections for the governor’s RhodeWorks project assumed a diversion rate of 25%, meaning that 25% of the expected tolls would not be collected because truckers would find alternate routes.

One needn’t hire an outside analyst like CDM Smith to conclude that the better part of any diversion will come from out-of-state truckers.  Just for illustration of the effects that change could have, if we apply the diversion evenly across all routes and assume the extreme that all diversion would come from out-of-state truckers, then the percentage of traffic more than flips.  In-state trucks would account for 60% of all truck traffic in the state.

Applying rough ratios for a multi-trip discount and assuming the discounts would all go to in-state truckers, they would still be responsible for nearly 57% of all tolls, or $34 million per year, if the goal is $60 million in revenue.  That’s more than $10 million larger than implied by CDM.

Remember, too, that there’s a spiral problem with this sort of data.  The lower the traffic, the higher the tolls, which means more traffic will divert.  (And don’t forget other sources of per-mile revenue that Rhode Island collects from truckers that would be affected by diversion.)