Almost all tolled highways and bridges employ some type of tiered pricing approach based on vehicle size and/or weight. The most common approach to classifying vehicles for toll pricing in the United States is based on the number of axles. Other less common methods include a combination of vehicle axles, weight, and a vehicle size based classification method known as profiling. While weight-based systems have lost favor due to high capital and maintenance costs, profile-based vehicle classification is gaining momentum, particularly in all electronic tolling and managed lanes environments.

Profile-based vehicle classification involves grouping vehicle sizes with lower and upper limits, which depend on vehicle length and height. These values then correlate to a predetermined pricing schedule. This system is most commonly achieved by placing laser scanners overhead on the tolling gantry and is similar to the radio frequency identification automatic vehicle identification antenna and image capture cameras utilized to support transponder and video tolling. While costs associated with the system components are roughly the same, the installation and maintenance costs are significantly less, and the need for extensive tolling zone infrastructure (conduit, pull boxes, special pavement, etc.) is eliminated. In addition, an overhead laser scanner can be easily replaced, but a damaged pavement loop system requires significant maintenance and can even affect traffic.

Profile-based vehicle classification is not ideal in every case. Some agencies have highly specific bond covenants that dictate this methodology based on findings in investment grade traffic and revenue studies. In this case, moving away from axle-based vehicle classification (explained in the next paragraph) could prove highly complex. Other agencies have long-standing pricing schemes based upon axle count, and the prospect of changing methods may be daunting. Conversion of an existing vehicle classification methodology must be well planned and requires significant public outreach. Vehicle profiling is often best suited for new or converted all electronic tolling facilities or managed lane facilities.

Axle counting is an open-road tolling system that involves placing several inductive loops, known as “smart” loops, just beneath the pavement surface. In many cases, a special pavement design is developed within the tolling zone under the toll gantry, and an extensive conduit and junction box arrangement is required to support the loops. Generally speaking, a loop-based axle counting solution in an open road tolling environment provides more accuracy than laser-based profile systems, usually by a few tenths of a percent. Both solutions achieve more than 99% accuracy in controlled testing environments. Business rules are often employed to ensure that customers are not overcharged while also protecting the agency’s toll revenue stream.

A pricing scheme that relies on vehicle size rather than a simple axle count is often more fair to customers. For example, an axle-based solution would charge the same toll for a dump truck that raises an axle when approaching the tolling point and a pickup truck towing a small utility trailer. A profile-based solution, however, would allow for a different, more equitable pricing approach based upon the distinct profile of the dump truck.

As more all electronic tolling facilities and managed lane implementations are deployed across the country, profile-based vehicle classification is gaining momentum and proving to be a sound alternative to more traditional approaches. Agencies and operators want to reduce capital costs, simplify maintenance, and eliminate costly infrastructure within the tolling zone while protecting toll revenue with a sound pricing scheme that customers will welcome. A profile-based vehicle classification system may be the solution to that issue.

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About the author

Kevin Palmer, PE, PMP
Kevin Palmer, PE, PMP
Kevin serves as RS&H’s Tolls Technology Leader and has a range of progressive experience in program management, consulting, and system planning. RS&H provides specialized solutions for the planning, scheduling, and testing of toll and managed lanes systems.