Heavy Trucks: Appendix 9
Use of Lane Restrictions Involving Trucks (E)
Lane restrictions fit under the umbrella of lane management. The FHWA Freeway Management and Operations Handbook (FHWA, 2003) defines managed lanes as:
The emphasis on efficiency is the original objective given by agencies for the introduction of lane restrictions involving trucks. The FHWA Handbook on Freeway Management and Operations (FHWA, 2003), however, also states that "the goals of various forms of truck lanes are to improve traffic operations, improve safety and facilitate the flow of goods." The document goes on to state that "truck lanes fall into the following categories:
Separate roadways for trucks, especially those relating to dedicated roadways and interchanges, will normally involve high capital investments and extended periods for design, construction and installation. Creating climbing lanes may also involve considerable expense, but will be less costly than separate truck roadways. Therefore, while these may be valid strategies for improving truck operations and safety, they are outside the scope of the Strategic Highway Safety Plan (which focuses on short term, low cost improvements) and hence are not considered appropriate for further discussion in this guide.
Implementation of lane restrictions (more appropriately termed "lane-use restrictions") on an existing highway or freeway, is a potential strategy for consideration when addressing the heavy truck emphasis area. However, because lane-use restrictions have generally been considered a means for improving operations, safety has been an after-thought at best, and for the most part has not been the primary driver where this strategy has been employed. Indeed, there are some who believe that under certain circumstances this strategy could have an adverse effect on safety. In any event, the effectiveness of implementing lane-use restrictions for trucks as a means of achieving improvements to the safety of a highway should be considered in the experimental stage.
Lane-Use Restrictions for Trucks
The applicability of lane-use restrictions is generally limited to sections of roadway with at least three lanes in one direction. This allows trucks to be restricted to the two right-most lanes, leaving one lane for truck-free operation. Most often, these are freeway facilities, including interstate routes.
The North Carolina DOT has implemented truck lane-use restrictions in a large scale experiment along 123 miles of interstate highway. Exhibit 2 is a schematic of the configuration used in North Carolina.
The North Carolina DOT identifies the following safety benefits from these type of restrictions:
NCDOT designated 123 miles of interstate with lane restrictions for trucks. Because of concerns over adverse safety effects, truck lane restrictions were excluded from highway sections with left-side exits and merging areas, and between closely spaced interchanges. NCDOT's Traffic Engineering and Safety Systems Branch is monitoring the effectiveness of the lane restrictions for impacts on crash rates, crash severity, and traffic operations. The engineering evaluations of the safety and operational performance of these restricted sections will be used to determine where to continue the lane restrictions, and where other countermeasures may be more appropriate. (North Carolina DOT)
The South Carolina DOT, working in cooperation with FHWA has been identifying strategies to apply to high-crash locations and segments on its freeway system. While many different strategies were tried, truck lane restrictions were considered in one location. SCDOT's efforts demonstrated the importance of working with stakeholders in developing an acceptable, appropriate countermeasure.
One problematic section of Interstate 85, a major north-south route for heavy trucks, had a continuing history of crashes. SCDOT's assessment of available national studies showed that lane restrictions potentially could lower the truck-involved crash rate.
When it became known that the State was considering such restrictions, the trucking industry expressed some concerns about safety and operations. To deal with the trucking industry concerns, FHWA and SCDOT implemented a pilot project to study lane restrictions. SCDOT established the restrictions temporarily for one year on two high-crash interstate segments. The South Carolina Department of Public Safety deployed targeted enforcement, both for lane violations and aggressive driving violations.
Following the implementation of lane restrictions and their targeted enforcement, the SCDOT observed a 78 percent reduction in truck-related crashes. This outcome enabled FHWA, SCDOT, the SC Department of Public Safety, and the South Carolina Truckers Association to reach a consensus that restricting trucks to the two right travel lanes on three-lane sections would offer improvements in safety and traffic operations. Truck lane restrictions were then expanded to an additional 170 kilometers (106 miles) of interstates in South Carolina.
We note here that the observed safety effect would relate to the combined program of lane use restrictions as well as enforcement. Moreover, it is not clear that the before-and-after study did not suffer from the common problem of 'regression to the mean'. Indeed, Public Roads magazine reported in 2003 that since the full implementation of truck lane restrictions in 2001, truck crash frequency increased slightly on interstates in South Carolina , but fatalities involving heavy trucks decreased. (Public Roads, 2003)
The Texas DOT has also expressed interest in applying this strategy. Legislation has been passed allowing The Texas Department of Transportation (TxDOT) along with local jurisdictions to restrict lane use by class of vehicle (i.e. trucks). Restricted truck lanes can only be enacted on highways with a minimum of three lanes in each direction, assuring that trucks will always have access to at least two lanes. Trucks may also use the left lane to pass another vehicle or enter and exit a highway. Truck lane restrictions have been implemented on I-10 E in Houston, SH 225 in Houston, Pasadena, Deer Park and La Porte, I-45 N in Houston and I-10 in San Antonio.
The Texas Transportation Institute (TTI) performed traffic studies before and after enactment of the restrictions on I-10E in Houston. According to the TTI study, although there are several factors which affect crash rates, the lane restriction likely had a role in reducing vehicle crashes by 68% along the freeway main lanes during the 36-weeks that were monitored.
The TTI study noted that the imposition of lane restrictions for trucks improves safety by reducing differential vehicle speeds, lane changes and passing maneuvers. TxDOT believes that the safest conditions exist when vehicles are not traveling at widely differing speeds. Removing the conflict of the differential speed between passenger vehicles and large trucks traveling in the left lane will reduce the number of lane changes and passing maneuvers attempted by the trailing passenger vehicles, thereby reducing the number of collisions. Large trucks generally take longer time to accelerate to the prevailing speed compared to passenger cars. The resulting speed differential tends to promote, lane changes as faster vehicles attempt to move around the slower moving trucks. (Moore, 2004)
The FHWA Handbook on Freeway Management and Operations offers the following additional bit of experience:
"A truck restriction lane on I-40 near Knoxville resulted in a substantial reduction in the percentage of trucks traveling the left lane even with minimal sign usage and enforcement." (FHWA, 2003)
The FHWA handbook also provides the following summary of experience with truck lane-use restrictions:
The Handbook, quoting from an earlier report, summarizes some anticipated impacts from these types of restrictions.
Cate and Urbanik (2004) performed a simulation study of various truck lane-use restrictions. They concluded that "As in previous research, the truck lanes are shown to have little effect on vehicle density, level of service, average speed, and average travel times on level terrain. However, the effect of truck lane restrictions on these measures on steep uphill grades (4%) is substantial.
To consider the safety impact of truck lane restrictions, the speed differential between cars and trucks and the total numbers of lane changes during the simulation period are examined. On level terrain, the speed differential between cars and trucks is affected by less than 1.0 mph in all scenarios. On 4% upgrades [without climbing lanes], the speed differential is increased by as much as 10 mph. This result would indicate a possible safety penalty to be paid for lane restrictions on upgrades. In addition, the average speed difference between the passing vehicle and the vehicle being passed is increased by a maximum of 0.6 mph. The apparent reduction in the total number of lane changes easily offsets any concerns raised by the slight increase in speed differential."
A simulation study of truck lane-restrictions was also conducted by Gan and Jo (2003). They concluded:
The simulation and other studies indicate that ramp merge and diverge areas are adversely affected by truck lane restrictions.
Overview of Issues Involved with Implementing a Lane-use Restriction Strategy
This strategy is considered as experimental as an approach to addressing truck-involved crashes. While the strategy has clearly been tried for many years and indeed may be considered widespread, its primary intent historically has been focused on traffic operational benefits.
Success implementing this strategy will depend on the ability to measure or observe an improvement in the safety of the facility. It will also depend on the acceptance of the strategy by users, including the freight industry, other users, and law enforcement officials.
Expected Safety Performance
The results and experience cited above are a combination of measurements, simulations, and perceptions of impacts on safety. The crash impacts cited may involve, in the case of analyses of crash data, naïve before-after comparisons, which do not account for other possible explanations for the change in crash experience. For instance, where high-crash locations are treated, the effect of 'regression-to-the-mean' may not have been accounted for. In some cases, enforcement was increased, which can have its own independent effect on traffic safety.
Simulation studies produce what the authors expect or surmise to be useful surrogates for safety (e.g., speed differentials and lane changes). These surrogates have tenuous, at best, documented evidence of a valid relationship with the number of crashes. Finally while opinions of road users are helpful to draw insights, they are just opinions. Therefore, one must be careful about conclusions drawn from the experience to-date. More well-designed, formal safety evaluations are needed. This is the reason that this strategy has been labeled as experimental. From a safety perspective especially, pilot studies are still considered the best approach, if an agency is interested in implementing truck lane-use restrictions.
In most cases, legislation will be required to authorize restriction of truck lane-use. In some cases the legislation may authorize both state DOT's and local agencies to apply truck lane-use restrictions on facilities under their jurisdiction. Legislators have also passed laws on restrictions for specific segments of specific facilities.
When implementing a lane restriction program, whether statewide or locally, it will be important to include all the stakeholders, especially law enforcement agencies and organizations representing commercial carriers using heavy trucks.
The opinions and insights of truck operators will be important to understand if the program is to have a good chance of succeeding. The following summary of a road-user survey in Puget sound was published on the internet by TranSafety, Inc, in their Auto and Road User Journal in 1997 (http://www.usroads.com/journals/aruj/9710/ru971001.htm).
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.
"Researchers administered opinion surveys to truck drivers and motorists regarding … three sections of highway…[on which truck lane restrictions had been implemented].
Some [selected] 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:
Restricting lanes in areas with a high proportion of merging and diverging traffic might not be safe for traffic operation."
Matthew Cate, Thomas Urbanik, Jr., Another View of Truck Lane Restrictions, Texas Transportation Institute, Presented at the TRB 2004 Annual Meeting.
FHWA, Freeway Management and Operations Handbook, Final Report, September 2003, Chapter 8, http://ops.fhwa.dot.gov/Travel/traffic/freeway_management_handbook/chapter8_01.htm
Public Roads, FHWA, November/December 2003 · Vol. 67 · No. 3, http://www.tfhrc.gov/pubrds/03nov/11.htm
Meg Moore, private communication, May, 2004
Albert Gan and Sijong Jo, Operational Performance Models for Truck-Lane(sic) Restrictions, Florida International University, Miami, FL, April 2003