The barrier system concept of toll operations has been used almost universally on all new US toll facilities built since the 1960s. In the early 1990’s the conventional barrier plaza was supplemented by non-stop express lanes employing ETC as the single method of payment, complemented by a license plate capture system for violators.
All toll collection points on the barrier system charge a flat fee to authorize use of a fixed length of a toll facility by a particular vehicle type. In the cases of bridges or tunnels and shorter toll roads, this single toll represents the fee to use the entire facility. Plaza location is a matter of economics and opportunity (i.e., where developable right of way exists and can be reasonably acquired across the mainline roadway section). In the cases of bridges and tunnels, unconventional location solutions, such as suspending the administration/plaza building from the underside of the bridge and abutting the plaza/administration building to the toll lanes, as is the case for the Tobin Bridge and Lincoln Tunnel, respectively, are sometimes necessary.
A barrier system will typically have one or more mainline plazas (or one pair for both directions of traffic) that charge the toll for a specific segment of the system. This segment would typically range from 10 to 30 miles. For example, a toll road financed on a 10¢ per passenger car mile basis and is 30 miles long could have two mainline barrier plazas which each charge $1.50.
Some barrier systems allow free movements on specific short segments between the barrier plazas. Most barriers systems, however, are “closed” in that they prevent free movements through the strategic location of interchange ramp toll plazas. Not all interchanges and ramp movements need to be tolled to meet this requirement. Plaza locations will typically be selected so that state departments of transportation (DOTs) and high volume and speed interchanges may be constructed without ramp toll plazas. For example, it is often desirable to avoid locating conventional toll plazas at an interstate-to-interstate interchange with multiple high-speed directional ramps. The Illinois Tollway designed the new I-355 / I-55 interchange (Exhibit 2-3) in this manner (i.e., I-355 has two barrier plazas at each end and the centrally located I-55 interchange does not have ramp toll plazas).
Location Requirements for Personnel Support
Mainline barrier plazas have traditionally been attended facilities, with various improvements for personnel assigned to work on the site including offices, parking, cash handling security, utilities, emergency generators, material, and equipment storage, and other personnel support features. Barrier system ramp plazas often operate unattended. These plazas still require some infrastructure and utility support (e.g., power and communications), but the scale of site improvements is substantially reduced, and therefore allow more flexibility in location than attended facilities.
Plaza Locations and Geometric Limitations
Because mainline plazas restrain or meter passage on high-speed routes, locations are generally selected where; the required right of way can be reasonably acquired, adequate stopping sight distance exists, and a sufficient length of roadway is on a tangent section. Geometric limitations are less than those encountered when locating ramp plazas, although constraints on design may be imposed by portal or approach features at tunnel or bridge crossings, as well as economic development along plaza rights of way.
Trumpet interchanges do not accommodate barrier system ramp plazas easily, primarily because two of the four typical movements through the trumpet interchange are not tolled, and so free lanes must be segregated from the toll lanes. Barrier system ramp plazas typically follow the interchange design, with the exception that individual ramps need to be several hundred feet longer and comply with stopping sight distance standards in advance of a small two- or three-lane ramp plaza.
Vehicle processing speeds are faster than in ticket-system exit lanes, and implementing a non-stop ETC lane requires less transaction processing. Nonetheless, interchange ramp designs must often be altered to provide additional tangent length and transverse space for locating a small toll plaza with some expansion capability.
Plaza Sizing and Growth Limitations
When the policy is to require vehicles to stop to pay the toll, plaza design must account for the required number of lanes based on a composite vehicle throughput, and the amount of approach pavement required for safe vehicle storage. Alternative plaza locations may need to be considered if the available plaza width is inadequate.
The addition of non-stop lanes adds new considerations. Dedicated lane design requires the provision for signing and pavement markings to keep dedicated lane approaches open, particularly when the cash lanes have long vehicle queues. Express lane design requires the physical separation between vehicles moving at high speed and vehicles either at rest waiting to pay the toll or decelerating while approaching the plaza. For plazas combining express lanes and a conventional plaza, the approach and departure length requirements become paramount. These plazas should be located at least one mile from the nearest interchange.
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