Road Injury Prevention Litigation Journal
Auto and Road User Journal
Copyright © 1997 by TranSafety, Inc.
October 15, 1997
TranSafety, Inc.
(U.S. and Canada)
(360) 683-6276
Fax: (360) 335-6402

Should Commercial Vehicle Traffic Lanes Be Restricted?

Restricting large-truck travel to specific lanes is becoming increasingly popular. Approval and awareness of truck-lane restrictions are related to concerns about traffic efficiency and safety, pavement wear, and motorist intimidation. Safety groups are increasingly pressuring transportation agencies to implement truck-lane restrictions to improve traffic safety and congestion. To gather more information, researchers developed profiles to measure the opinions of both commercial truckers and motorists about truck-lane restrictions. Transportation agencies can use this information to identify resistance to truck regulations and target drivers who need to be aware of them.


Researchers Jodi Koehne, Fred Mannering, and Mark Hallenbeck reported the results of this study in "Analysis of Trucker and Motorist Opinions Toward Truck-Lane Restrictions." The paper appeared in the Transportation Research Board's Transportation Research Record No. 1560, Safety and Human Performance, published in 1996.

This study focused on trucker and motorist opinions of the truck-lane restrictions in the Puget Sound (Washington State) area. The researchers pointed out the importance of understanding these opinions, because "they form the basis for political pressures that can doom even the most well-thought-out and effective truck restriction policy."

A popular option for increasing roadway efficiency is regulation of large-truck travel. The major types of restriction imposed on truck travel are lane, route, time-of-day, and speed. Restrictions have been justified for these objectives:

  • to improve operations,
  • to improve the levels of safety,
  • to provide for more even pavement wear, and
  • to ensure better operation and safety through construction zones.


As a response to public concern, three zones in the Seattle metropolitan area currently include truck restrictions. All are in one direction only and on an ascending grade.

Two sections of Interstate 5 restrict large-vehicle traffic. The first restricts two of the southbound lanes near Sea-Tac Airport. Here, an added HOV lane caused narrower general-purpose lanes and shoulders. In 1992, trucks were banned from the two left (fast) southbound lanes. Southbound travel on this Southcenter Hill section of highway was more than 90,000 vehicles per day for 5 km (3 miles), with an ascending grade that at times exceeds 4 percent. The wording of the regulatory signs is "Vehicles Over 10000 GVW Prohibited Left 2 Lanes--Except Transit."

Another restricted area of I-5 southbound is an ascending grade approximately 3 km (2 miles) long near the city of Tacoma. Restrictions began here in the late 1980s. As many as five lanes carry about 75,000 vehicles per day through a series of curves and on-and-off ramps that require frequent merging and diverging. The authors noted, "Accident- and incident-related congestion on this portion of highway is regarded as among the worst in the state." The wording of the truck regulatory signs on this highway section is "No Trucks Left Lane."

State Route 520, in a section near the Seattle suburb of Redmond, includes a grade as steep as 5 percent and carries a traffic volume of 40,000 vehicles per day on three lanes in the restricted direction. A major on ramp in the middle of the grade complicates traffic flow. The regulatory signs posted on this section also read "No Trucks Left Lane."

The Washington State Highway Patrol does not enforce these lane restrictions; motorists and truckers have not been made aware of this.


Researchers administered opinion surveys to truck drivers and motorists regarding the three sections of highway described above.

Participants in the truck survey included local drivers and interstate drivers who visit the Puget Sound regularly. They participated in the surveys at truck stops.

For the motorist survey, the license numbers of cars traveling the affected highway sections were collected, and questionnaires were mailed to the owners.


Some findings from the truckers' survey responses were:

A relatively small number of truckers favored keeping the restrictions. They gave several reasons:

Some findings from the motorists' survey responses were:

An interesting point is that 32 percent of the truckers thought that the restrictions (which are voluntary) were being enforced, while only 3.17 of the motorists thought so.


Truck-Lane Restriction Favorability

Researchers analyzed factors that caused survey respondents to approve of truck-lane restrictions. Such factors included:

The research showed 99.99 percent confidence that the two groups--truckers and motorists--should have a meaningful difference in their opinions about truck-lane restrictions.

Trucker disagreement with the restrictions showed:

Drivers on the I-5 Southcenter Hill and SR 520 sections were more in favor of restrictions, possibly showing that they perceived a higher accident risk than did drivers on the I-5 Tacoma section--which has more lanes.

Motorist favorability toward lane restrictions showed:

Truck-Lane Restriction Awareness

The researchers were surprised to learn that less than one- third of the surveyed motorists knew about the truck-lane restrictions. Their statistical model shows:

Researchers cautioned that, because survey respondents tended to be older men, those with strong opinions about trucks and their safety issues may be overly represented.

The researchers suggested four explanations for the finding that drivers on the I-5 Southcenter Hill and SR 520 sections were more likely to be aware of the restrictions than drivers on the I-5 Tacoma section:

  1. Sign placement may be better on the Southcenter Hill and SR 520 sections.
  2. Lane restrictions on the I-5 Tacoma section have been in effect longer and may have been forgotten by or become too familiar to motorists.
  3. The I-5 Tacoma section, which includes a lot of merging and diverging traffic, may have distracted motorists from the truck-lane regulatory signs.
  4. The Southcenter Hill and SR 520 sections have fewer lanes than the Tacoma section, with more trucks using the left lanes and increasing motorists' awareness of truck-lane issues.

Implications of Findings

Based on their statistical models for favorability and awareness, the researchers recognized several important points.

  1. Trucker profile

    When promoting the concept of lane restrictions, transportation agencies should target the group of truckers identified as least likely to favor them.

  2. Areas to restrict

    Restricting lanes in areas with a high proportion of merging and diverging traffic might not be safe for traffic operation.

  3. Improving motorist awareness

    Transportation agencies should target their marketing and motorist awareness efforts toward drivers who do not fit the motorist profile for favoring truck restrictions.

Copyright © 1997 by TranSafety, Inc.

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