Method and data

The Michigan crash data were supplemented by adding a GVWR classification of the trucks to the record for each vehicle. The GVWR class of each CMV was determined from VINs. The VINs were decoded by David Hetzel of NISR, Inc. Of the 73,000 CMVs in the Michigan crash data, VINs for 38,703 could be decoded and assigned to GVWR classes 3 through 8. Table 31 provides the weight range for each GVWR class. Public Act 231 exempted class 3 to 6 trucks from certain regulatory requirements. These trucks will be termed medium-duty trucks or medium trucks in this analysis. Trucks with a higher GVWR (class 7 and 8) are referred to as heavy trucks.

Table 31 GVWR Class and Weight Range

Truck type GVWR class GVWR range (lbs.)

Medium-duty Class 3 10,001 to 14,000 Class 4 14,001 to 16,000 Class 5 16,001 to 19,500 Class 6 19,501 to 26,000 Heavy-duty Class 7 26,001 to 33,000 Class 8 33,001 or more

There did not appear to be any systematic bias to the truck records that could be decoded and assigned as medium or heavy. Distributions of the decoded records were compared with the same distributions for all CMVs and found to be similar. CMVs with known and unknown VINs were compared for crash severity, time of day, light and weather condition, crash type, and day of week. In general, the

distributions differed only by a few tenths of percent. It was therefore concluded that missing data on VIN does not bias the results in any material way.

Carriers operating the trucks were classified as interstate or intrastate using data in the Michigan crash file. This information was collected on the UD-10 as part of the supplemental data collected for the MCMIS crash file. This data element records whether the carrier had interstate operating authority, not whether the particular trip at the time of the crash was interstate or intrastate.(FMCSA 2006) Using the carrier type classification and the categorization of the trucks as medium or heavy, it was possible to identify in the Michigan crash data the medium-duty intrastate trucks specified in Public Act 231.

Table 32 shows the distribution of the decoded trucks by GVWR size and operating authority of the truck. Heavy/interstate trucks accounted for over 50% of the total number of trucks classified. The

Page 54 Medium/intrastate trucks

Medium/interstate and heavy/intrastate accounted for 8.1% and 28.7%, respectively. The sample sizes here were sufficient to provide statistically-reliable comparisons.

Table 32 Classification of Trucks by Size and Operating Authority Size/authority N Percent Medium intrastate 4,478 11.6 Medium interstate 3,149 8.1 Heavy intrastate 11,125 28.7 Heavy interstate 19,944 51.5 Total 38,696 100.0

The Act affects the regulation of medium/intrastate trucks in four ways: 1) they are not required to obtain or display a US DOT number; 2) they are exempt from hours-of-service regulations; 3) they are exempt from regulations related to knowledge of and compliance with the inspection or maintenance of

commercial motor vehicles; and 4) they are exempt from the requirement to provide certain safety-related documents to enforcement personnel.

Exemption from HOS and the vehicle inspection and maintenance requirements regulations have the most direct relationship to traffic safety. The HOS regulations are intended to reduce fatigue in truck drivers and fatigue-related truck crashes. More broadly, the HOS regulations are intended to help drivers remain alert and attentive to the driving task, and thus reduce crashes overall.

Vehicle inspection and maintenance regulations are intended to ensure that trucks are mechanically sound, and to reduce crashes related to defective equipment. Recent studies have shown that violations of the FMCSRs related to vehicle condition were related to higher crash rates. In a study that included the relationship of safety and compliance with various FMCSRs, vehicle condition was one of the factors most strongly correlated with crash rates. (Green and Blower 2011) Another study showed that violations of the brake adjustment criteria were related to higher crash risk. (Blower and Green 2009)

The first part of the analysis presents descriptive statistics on crash types and crash circumstances. All of the data presented are in the form of percentages of crashes. Frequencies are not shown because only a subset of the vehicles, about 38,000 out of 73,000, had sufficient information to decode VINs and classify vehicles as either medium or heavy trucks. Reporting frequencies for this subset could be confusing. Moreover, it is not necessary because the critical point is to compare the crash experience of medium and heavy trucks, not raw frequencies.

Overall, the distribution of crash severity was roughly similar for the four groups of trucks. (Table 33) Heavy trucks operated by interstate carriers had a higher proportion of fatal involvements compared with the other categories, but within the category of medium-duty trucks, intrastate trucks were reasonably similar to interstate, although the latter had somewhat higher proportion of injury involvements and the former somewhat higher proportion of PDO involvements.

Medium/intrastate trucks Page 55

Table 33 Percent Distribution of Crash Severity by Truck Size/Authority Michigan 2006-2011 Crash severity Medium intrastate Medium interstate Heavy intrastate Heavy interstate Fatal 1.0 0.9 1.1 1.6 Injury 19.0 22.1 17.7 20.4 PDO 80.0 77.0 81.3 78.0 Total 100.0 100.0 100.0 100.0