UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY

2565 PLYMOUTH ROAD ANN ARBOR, MICHIGAN 48105

- 2498

MEMORANDUM

SUBJECT: Truck and Trailer Roof Height Match Analysis

FROM: Amy Kopin

TO: The Docket

DATE: August 9, 2010

T he aerodynamic performance of a combination truck is dependent on many factors, including

tractor and trailer size and design, the gap between the tractor and trailer, vehicle speed, the height difference between the cab and trailer roofs, and many others. To assess the aerodynamic performance of combinations trucks more accurately, certain aspects of the truck need t o be defined, including the use of a standardized t railer during testing

. We are proposing that the

standardized trailer be determined by the roof height definition of the regulatory subcategories, including low, mid

, and high roof designs

.

Many tracto rs have roof fairings installed to improve aerodynamic performance. The height of the roof fairing is designed to minimize the height differential between the tractor and typical

trailer to maintain a smooth, attached airflow over the vehicle and reduce t he air flow disruption

, or drag

. Low roof tractors are designed to carry platform

- type trailers, which can include flatbed

or low

- boy trailers. Mid roof tractors are designed to carry tanker

- type trailers, which can include liquid and other bulk trailers

. High roof tractors are designed to pull box

- type trailers

, which can include dry van trailers and refrigerated van trailers

. However, we recognize that during actual operation tractors may pull trailers that do not provide the optimal roof height that matches the tractor and reduces drag . In this memorandum, this situation will be referred to as a
“mismatch

. ” In order to assess how often truck and traile r mismatches are found , we conducted a study based on observations of traffic across the U.S . The purpose of this memo is to describe how this study was carried out, and to present the results.

The intent of the study was to sample traffic in various locations throughout the U.S. Originally,

it was thought that truck weigh stations might provide goo d sources of this information, however, the best way to assess the h eight match of tractors and trailers currently in use would be through individual counts in person , and there was not sufficient data from truck stops to determine the trailer height

. To avoid the need to physically travel to different sites across the country to count

trucks , which would be cost prohibitive

, we determined it was possible to sample traffic through OFFICE OF
AIR AND RADIATIONthe use of traffic cameras. Nearly every state has a number of traffic came ras available for public

viewing

. Some states offer a large number of available cameras for their drivers and visitors to

view ; Figure 1 shows an example of the numerous available traffic cameras in Oregon.

Figure 1 Example of Av ailable Cameras

- Oregon

Source: http://www.tripcheck.com/Pages/CamerasEntry.asp

In many cases, cameras are close enough to the road to easi ly determine if the tractor and trailer roof heights match ; only cameras offering these close

- up views were used in this study

. Each camera used in this study was carefully selected to provide a clear view of the truck and trailer roof height ; an example is shown below in Figure 2 . Cameras were observed between 9 AM and

6 PM as long as cle ar pictures could be observed (i.e. sufficient daylight was available to s ee the

truck and trailer roofs and/or cle ment weather conditions were present that did not obscure camera views). D ata was taken in the last week of January, 2010 an d continued thro ugh the first week of February , 2010

. The data was taken such that at least one hundred consecutive truck and

trailer combinations were recorded from each cameraFigure 2 Sample Image from an Oregon Camera

The study sampled i mages from 33 real

- time traffic cameras , collecting d ata on more than 4

, 000

tractor

- trailer combinations

. Where possible, traffic passing in both directions was used in th is study. The process consisted of keeping count of whether or not the tractor and trailer roof height profiles matched . A combination was considered to be a match if the height of the tractor

roof was within a few inches of the height of the trailer roof, for example: if a tall trailer was hauling a tall container (as shown in Figure 2 ) , a medium roof tracto r was hauling a tanker, and a low roof tractor was hauling a flatbed trailer. I f the combination didn’t match, a note was made

as to what the combination was (e.g. high roof tractor with flatbed, etc.) Not es were also made if

a tractor was empty (not hauling a trailer) or was hauling an empty flatbed trailer. The locations used in this study cover 22 states across the United States , and a number of main highways , seeFigure 3

. Ac ross this broad geographic range, we found that the cameras are located within

predominately well

- populated areas, since the primary intent of these cameras is for the public to

have access to real

- time traffic congestion data , which is typically not a pro blem in less well

populated areas.Figure 3 Location of Cameras Where Traffic was Observed

Based on the data gathered, a pproximately 95% of trucks were “ma tched” per our definition (e.g.

b ox trailers were pulled by high roof tractors and flatbed trailers were pulled with low roof tractors). In general, the occurrence of a truck being lower than the trailer (e.g. day cab and tall container) was over seven times as likely as a truck being higher than the trailer (e.g. high roof truck and flatbed trailer). The amount of mismatch varied de pending on the type of location surveyed , for example o ver 99% of the tractors were in matched configuration s in Indiana at the

I

- 80/I

- 94/ I

- 65 interchange, which is representative of long

- haul o peration. On the other ha n d, in metro New York City, the match percentage was just over 90%

, in which case every mis match ed combination consisted of a day cab and a tall container

, Figure 4 shows the approximate

locations of thes e traffic cameras.

Figure 4 Map of Lowest and Highest Mismatch Percentage Found in this StudyIn addition, this study also looked at the number of trucks without trailers or trucks hauling

empty flatbeds, although the number of empty box trailers is unknown. It was found that of all the data included in this study (match and mismatch counts) that approximately 3% of the tractors were traveling without a trailer or with an empty flatbed ; the ratio between no trailer and empty fl atbed trailer wa s approximately 50 percent

.

References:

STATE WEBSITE

Alabama www.dot.state.al.us

Arizona www.az511.gov

California http://video.dot.ca.gov

Colorado www.cotrip.org

Del a ware www.deldot.gov

Florida www.fl511.com

Iowa http://hb.511ia.org/main.jsf

Kansas http://www.kcscout.net

Kentuc k y/Ohio www.artimis.org/camaccess1.html

Maine http://maineturnpike.com/traffic_cam s/traffic_cams.php

Massachusetts http://www1.eot.state.ma.us/
Michigan http://mdotwas1.mdot.state.mi.us/public/drive/rtt.cfm#1

Minn esota http://www.dot.state.mn.us/tmc/trafficinfo/cameras_map.html
Mississippi www.mstraffic.com

New York http://cameras.nysdot.gov/

New York http://peacebridge.com

North Carolina http://www.greensboro

- nc.gov/departments/GDOT/trafficinfo/

Ohio http://buckeyetraffic. org
Oklahoma www.oktraffic.org Oregon http://www.tripcheck.com

Tennessee http://www.tdot.state.tn.us/tdotsmartw ay/

Texas http://amaits.dot.state.tx.us/AMA

- ITS/default.htm

Texas www.transguide.dot.state.tx.us