NCHRP 20-07 Task 303

DIRECTORY OF SIGNIFICANT TRUCK SIZE AND WEIGHT RESEARCH

AASHTOSubcommittee on Highway Transport

June 29, 2011

MOTIVATION

Decision-makers are challenged to identify/utilize state of knowledge from truck size and weight research

• Extensive volume of related research, dating backnearly 80 years

• Breadth of related topic areas including infrastructure preservation, enforcement, safety, congestion, environment, industry costs,modal share, etc.

• Diverse interests ofpublic and privatesponsoring agencies/organizations

OBJECTIVE

To develop a “directory” of significant truck size and weight research that is brief, well-organized, and neutral

• Address the breadth of all related topic areas

• Consider research performed by various sponsoring agencies

• Be organized topically with embedded cross-references directing readers to additional findings

• NOT be inclusive of all related research

Only essential information will be included. Each finding will be carefully cited to support additional information gathering.

TECHNICAL APPROACH

Task 1: Distinguish Relevant, Significant, Useful Research from Body of Research• Timeliness

• Soundness of the methodology

• Scope and comprehension

• Conducted in response to an expressed need

Task 2: Extract Key Research Findings• Methodological strengths or shortcomings that may

influence validity of findings

• Trends or commonalities among publications that suggest a higher level of confidence

• Conflicting findings that challenge decision-making

Task 3: Produce Final “Directory of Significant Truck Size and Weight Research”

PROJECT PROGRESS

TOPIC

Infrastructure Preservation - Pavements

Draft - May 2011 Infrastructure Preservation - Bridges

Modal Share

Enforcement

Draft - Jun 2011 Highway Safety

Highway Geometrics

Industry Costs

Economic Impacts

Infrastructure Financing

Highway Congestion

Environment

Public Opinion

PRELIMINARY FINDINGS

Infrastructure Preservation – Pavements

• Higher axle weights can significantly increase pavement damage/costs– 3-4 exponent power for flexible

– 11-33 exponent power for semi-rigid/rigid

• Higher GVWs spread over more axles do not increase pavement damage and can even result in cost savings

• Multi-axle groups are– Less damaging than single axles for flexible pavements

– Unconfirmed for rigid pavements

• Increased axle spacing within a group– Increases fatigue damage to flexible pavement

– Decreases damage to rigid pavements

PRELIMINARY FINDINGS

Infrastructure Preservation – Pavements

• Pavement damage/costs vary by road classification and pavement design– Example: 5-axle, 80,000 lb tractor semitrailer causes

• $0.09 damage/mile on rural Interstate Highways

• $5.90 damage/mile on rural local roads

• Pavement damage/costsvary seasonally; potentialfor damage is highest during the spring

PRELIMINARY FINDINGS

Infrastructure Preservation – Bridges

• Bridge safety concerns– Overstress—bridge has inadequate load-bearing capacity

to accommodate loads (focus of research)

– Fatigue—bridge suffers from reduced life attributable to repeated loadings (generally limited to steel structures/ bridge decks)

• Bridge stress– Is affected more by total load than number of axles; GVWs

>80,000 lb can be used without excessive stress

– Increases with axle group weight

– Decreases with separating distance except on some continuous bridges with long spans

PRELIMINARY FINDINGS

Infrastructure Preservation – Bridges

• Bridge upgrade costs may be exaggerated– Includes existing deficiencies

– Assume full replacement over cost-effective alternatives

• Strengthening the bridge (when viable)

• Restricting select truck configurations indefinitely along non-essential routes

• Federal Bridge Formula has been criticized for– Setting overly cautious limits for shorter truck weights

– Allowing too much extra weight with additional axles

– Allowing trucks that could overstress continuous spans (originally considered stresses on simple-span bridges)

PRELIMINARY FINDINGS

Modal Share• Complex, profit-driven, economic relationships– Increased truck limits lead to lower truck transport costs

– Industry competition and regulatory pressure translates lower costs into lower transport rates

– Rail industry must either match lower rates or lose freight traffic—in either instance, rail revenues will decline

• Estimates of traffic diversion/rail revenue losses are highly variable and sensitive to– Regional commodity movement/infrastructure conditions

– Extent of truck payload capacity increases

– Evaluation assumptions

PRELIMINARY FINDINGS

Modal Share

• Shippers consider price and service – Rail service is almost always less expensive

– Truck service offers faster, door-to-door delivery

– For low-value commodities—such as coal, grain, or chemicals—price is often a priority over service

• Highest levels of modalcompetition exist for– Intermodal operations

with combined truck/rail transport

– Carload operations thatutilize boxcars

PRELIMINARY FINDINGS

Enforcement

• Challenged to accurately relate enforcement and compliance– Reliable estimates of illegal activity are lacking

– Wide divergence in U.S. enforcement practices

• Higher enforcement levels generally associated with higher compliance– Violation rates for fixed Interstate weigh stations

• <1% when enforcement is present• 15% when it is not

– Violation rates along bypass routes and/or using mobile enforcement are higher in frequency (30%) and magnitude

PRELIMINARY FINDINGS

Enforcement

• A combination of fixed and mobile enforcement can be most effective

• A greater reliance on technology provides increased spatial/temporal coverage

• Laws/regulations that are complex or containnumerous exceptionschallenge enforcementand subsequent prosecution

PRELIMINARY FINDINGS

Highway Safety

• Larger, heavier trucks can affect highway safety by – Increasing/decreasing truck traffic volumes

– Changing vehicle design/performance

– Causing a shift to alternate road classifications with higher/ lower crash rates• Interstates/Turnpikes are generally safer irrespective

of truck size or weight

• Inadequate crash/exposure data limit ability to relate truck size and weight to highway safety

PRELIMINARY FINDINGS

Highway Safety

• Larger, heavier trucks generally have– Lower crash rates (attributable to fewer truck trips)

– Higher crash severities

– Same or slightly higher crash risk based on vehicle handling and stability characteristics

• Double trailer trucks prone to rearward amplification

• Higher centers of gravity increase potential for rollover or ramp-related crashes

• Truck configuration findings are inconsistent– Double trailer trucks have higher/lower/same crash rates

and severities

– LCVs have higher/lower crash rates and severities; recent research suggests superior safety performance

PRELIMINARY FINDINGS

Highway Geometrics

• Design features most affected– Horizontal curves– Intersection turning radii– Passing sight distance– Sight distance at intersections/railroad grade crossings– Ramp interchanges

• Increased trailer lengths increase vehicle off-tracking

• Wider trucks on rural 2-lane highways elicit undesirable/unsafe actions by oncoming drivers

• Upgrade costs are highly variable and depend upon– truck configuration

– extent of roadway network to be redesigned

NEXT STEPS

TOPIC STATUS

Industry Costs

In progress (Jul 2011)Economic Impacts

Infrastructure Financing

Highway Congestion

Forthcoming (Aug 2011)Environment

Public Opinion

Final Directory of Truck Size and Weight Research

Forthcoming (Aug 2011)

QUESTIONS