New Hampshire Turnpike from Federal Truck Weight Limits Safety Analysis

ME DOT, Jan 27, 2005

“Geo-coded” crash data was available from the MDOT that could be used to analyze TST
combination truck crashes by functional highway class in Maine. A previous study of truck size
and weight noted a strong correlation between crash rates and functional highway class:

“Numerous analyses of crash data bases have noted that truck travel, as well as all vehicle
travel, on lower standard roads (that is, undivided, higher speed limit roads with many
intersections and entrances) significantly increases crash risks compared to travel on Interstate and other high quality roadways.

The majority of fatal crashes involving trucks occur on highways with lower standards…. The [fatal crash] involvement rate on rural Interstate
highways is 300 percent to 400 percent lower than it is on other rural roadway types and is
generally the same for all vehicle types.”‡

The geo-coded crash analysis divided 14,244 road segments in the Maine portion of the study
network into 3 groups of roadways (each network segment was in one, and only one group):

Non-Exempt Interstates, controlled-access facilities expected to gain traffic in the scenario
under study (i.e. exempt weights allowed on the interstate). 546 centerline miles (of two or
more lanes, running in the same traffic direction).

Maine Turnpike, controlled-access facilities expected to lose traffic in the study scenario - 42
centerline miles; and,

Diversion Routes, which constitute the rest of the study network, and which are expected to lose
traffic, under an Interstate exempt scenario - 4,538 centerline miles (primarily of two lanes,
each running in opposite traffic directions).

Exhibit 11: Annual Network TST Crashes
Three years (2000 – 2002) of geocoded crash data were filtered by recorded vehicle type to extract only crashes involving 5 or 6 axle TST
vehicles, with GVW registrations of 80,000 lbs. or more, occurring on some portion of the study network. A total of 1,000 crashes from the three years of data passed both filters to constitute the
crash sample.

Exhibit 11 shows the annualized number of 5 and 6 axle TST crashes on the Maine Turnpike, non-exempt Interstate, and study network “diversion” routes.

A process was applied that linked the estimated TST average annual daily traffic (AADT) by road
segment in the study network to TST crashes on those segments.

The process allowed the study
team to estimate “crash rates” expressed as TST crashes per “100 million vehicle miles traveled”
(HMVMT) by type of highway facility in the study network.