Comprehensive Truck Size and Weight Limits Study: Comparison of Results Report
Chapter 4: Highway Safety & Truck Crash Comparative Analysis
4.1 Purpose
The purpose of this section is to compare principal results of the Safety Comparative Analysis with other similar studies available in the literature. This involves two main objectives. First, those documents summarized in the revised desk scan that contain quantitative results pertaining directly to enforcement costs and effectiveness (i.e., the main objectives of the current 2014 CTSW Study) are identified. Second, the results from each of the selected documents are reviewed and objectively compared with the results of the 2014 CTSW Study. Two types of comparisons are provided: (1) those pertaining to the scenario results; and (2) other CTSW Study results.
4.2 Comparison of Safety Study Findings
The Safety Comparative Analysis estimates impacts on the costs and effectiveness of truck size and weight (TSW) enforcement for the six 2014 CTSW Study scenarios.
Table 4-1 summarizes the findings of several key crash studies conducted over the last 20+ years. One can quickly see that there are no findings for LCVs, only for single and double combinations. This is because in all the studies there persisted this issue of a lack of sample size and data detail for LCV crashes. A few studies had results for triples or other double combinations, but review of the reports revealed that the sample size of annual crashes was 20 or less. The safety team also opted not to include the findings of the study by Dr. Sowers, as Dr. Dan Blower profoundly critiqued this research.
A first comparison can be made of the internal consistency of the 2014 CTSW Study estimates for singles and doubles. While the rural and urban Interstate rates vary from state to state, the rural Interstate rates are around 0.5 or less for singles, close to the rate for doubles in Kansas. The 2014 CTSW Study did not compute crash rates for doubles in Washington, Idaho and Michigan because it was not part of the scenario to do so. These results are similar to those of Abdel-Rahim using data from some of the same states, but in earlier years. The Western Uniformity Scenario Study has higher rates for both singles and doubles; it is difficult to say why, but that study drew crash data from many more states, so the many state-level differences (e.g., reportability thresholds; data collection practices) may be at play. It is not possible to say much more.
It is more difficult to include the work by Campbell et al. in the comparison because the work involves fatal crashes only. The differences with respect to operating environment are generally the same with urban interstates have high rates then rural interstates. So, what can we conclude? The safety team has some reasonably consistent crash rate estimates for doubles and single combinations, but there is virtually no information on LCVs. Table 4-1 provides yet additional evidence of the need to enhance and fundamentally re-think how we address the safety implications of larger and heavier trucks.
There are really no comparisons to be made for the inspection and violations analysis as no studies of that type were discovered in the literature.
| Study | Crash Data Source | Exposure Data Source | Findings (Crashes per million vehicle miles) | Comments |
|---|---|---|---|---|
| Jovanis et al., 1989 | Fleet records; all crashes | Fleet dispatches for routes with both twins and 3-S2 operations |
3S2*: 3.83
Twin*: 3.52 |
Data from one carrier; all crashes |
| Campbell et al., 1988 |
TIFA
(1980-84) |
NTTIS
(1985) |
Single*: Rural 4.50
Urban 5.80 Double*: Rural 4.06 Urban 4.30 |
From Western Uniformity Study Table VII - 7, Page VII - 17 |
| 2014 CTSW Study |
Washington
(2008-2011) |
WIM and FHWA VMT |
Single: Rural 0.27
Urban 0.35 Combined: 0.31 |
Crash frequencies per year range from 85-100 in Idaho, to 270 in Michigan
Doubles sample sizes small in Washington, Idaho and Michigan |
|
Idaho
(2008-2010) |
WIM and FHWA VMT |
Single: Rural 0.47
Urban 0.67 Combined: 0.51 |
|
|
|
Michigan
(2008-2012) |
WIM and FHWA VMT |
Single: Rural 0.19
Urban 0.24 Combined 0.22 |
|
|
|
Kansas Turnpike
(2008-2012) |
VMT
(2008-2012) |
Single: Rural 0.58
Urban 1.00 Double: Rural 0.46 Urban 0.53 |
Crash frequencies ranged from 50 to almost 80 per year |
| Abdel-Rahim et al., 2013 |
Utah
(1999-2004) |
FHWA and WIM |
Singles: 0.48 to 0.81 per year
Twin: 0.48 to 1.06 per year |
Only computed crash rate per year all facilities; no route type breakdown |
|
|
Idaho
(2003-05) |
|
Single: 0.78 to 0.92 Double: 0.91 to 1.16 |
Only computed crash rate per year all facilities; no route type breakdown |
|
Western Uniformity Scenario
(1995-99) |
Crash data from 13 WUSA States | VMT for study using FHWA VMT |
Rural Inter.: 1.50 single
1.83 multi Urban Inter.: 2.10 single 1.39 multi |
|
* Include all crashes for firm, not just DOT reportable
** These rates are fatal involvement rates per 100 million vehicle miles
