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

The 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 to be 

defined, including the use of a standardized trailer 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 tractors 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 the 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 trailer 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 good sources of this information, however, 

the best way to assess the height 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 RADIATION
the use of traffic cameras. Nearly every state has a number of traffic cameras 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 Available Cameras - Oregon

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

In many cases, cameras are close enough to the road to easily 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 clear pictures could be observed (i.e. sufficient daylight was available to see the 

truck and trailer roofs and/or clement weather conditions were present that did not obscure 

camera views). Data was taken in the last week of January, 2010 and continued through 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 camera 
Figure 2 Sample Image from an Oregon Camera

The study sampled images from 33 real-time traffic cameras, collecting data on more than 4,000 

tractor-trailer combinations. Where possible, traffic passing in both directions was used in this

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 tractor was hauling a tanker, and a 

low roof tractor was hauling a flatbed trailer. If the combination didn’t match, a note was made 

as to what the combination was (e.g. high roof tractor with flatbed, etc.) Notes 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, see 
Figure 3. Across 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 problem in less wellpopulated areas.
Figure 3 Location of Cameras Where Traffic was Observed

Based on the data gathered, approximately 95% of trucks were “matched” per our definition (e.g. 

box 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 depending on the type of location

surveyed, for example over 99% of the tractors were in matched configurations in Indiana at the 

I-80/I-94/I-65 interchange, which is representative of long-haul operation. On the other hand, in 

metro New York City, the match percentage was just over 90%, in which case every mismatched

combination consisted of a day cab and a tall container, Figure 4 shows the approximate 

locations of these traffic cameras.

Figure 4 Map of Lowest and Highest Mismatch Percentage Found in this Study
In 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 flatbed trailer was 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

Delaware www.deldot.gov

Florida www.fl511.com

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

Kansas http://www.kcscout.net

Kentucky/Ohio www.artimis.org/camaccess1.html

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

Massachusetts http://www1.eot.state.ma.us/

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

Minnesota 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/tdotsmartway/

Texas http://amaits.dot.state.tx.us/AMA-ITS/default.htm

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