Dedicated Lane Design Issues
Existing design standards have provided little guidance on the subject of dedicated lane design, except for some work by the Institute of Transportation Engineers. Current AASHTO or MUTCD guidelines do not offer guidance in the design of dedicated lanes. Some important facts regarding the evolution of ETC dedicated lanes are the following:
Non-stop lanes make the drivers’ use of a toll plaza more difficult, in that more decisions are required to navigate the plaza. In the case of many survey respondents, ETC dedicated lanes represent a third type of toll lane, which may be further complicated by other restrictions, such as cars or trucks only. Consequently, the driver’s attention is often drawn to canopy signs, lane use signals and pavement markings, and much less to nearby vehicles. The case study by Mohamed, Abdel-Aty and Klodzinksi(1) reviews how conflicts in lane selection can lead to increased accidents.
Differential speeds resulting from differences in non-stop and stop-and-go traffic flow corresponding to dedicated lane and cash lane usage, respectively, needs to be minimized within the approach and departure zones.
1. Safety Considerations in Designing Electronic Toll Plazas: Case Study,” ITE Journal, March 2001, Mohamed, Abdel-Aty and Klodzinski .
Dedicated Lane Guideline Development
Good design of a toll plaza with dedicated lanes should not only consider applicability of the individual guidelines from each section, but should view the guidelines collectively to derive symbiotic benefits that exceed the benefits derived when the guidelines are individually adopted (i.e., the whole exceeds the sum of the parts).
With ETC dedicated lanes now able to process the majority of traffic in peak traffic periods; the sometimes marginal benefit-cost ratio associated with automatic coin machines (ACM) support consideration of converting ACM lanes to ETC dedicated lanes. Coin machine lanes were developed long before development of ETC technology, and were intended to increase vehicle processing without incurring a continuous operations labor cost. Now, ETC dedicated lanes far exceed the original objectives of coin machines. The elimination of coin machine lanes, when feasible, reduces the number of lane types a driver can select during the approach to the toll plaza. Consequently, drivers can spend more time viewing their surroundings and be better prepared to take defensive measures to avoid potential incidents.
Some operational constraints at bridge and tunnel crossings require unusual measures, such as partial facility closures or contra-flow traffic operations during maintenance periods such as tunnel cleaning or bridge deck repairs. During these periods, existing pavement markings and signing may not apply, resulting in sometimes complex traffic control and guidance that may not be anticipated by drivers intending to use a bridge or tunnel crossing. In addition, at facilities with limited plaza space and prone to bottlenecks, a case could made for implementing only mixed-use toll lanes that include both ETC and cash operations.
Pavement markings can be used to separate dedicated lanes from cash lanes for a length of approximately one half of the queue zone. This not only follows the intent of the AASHTO recommended lane separation, but also helps to identify a boundary to avoid straying into by vehicles waiting in queues.
Dedicated Lane Guidelines
Based on the survey results, observation of existing practices, and published research literature(2), the following dedicated lane guidelines are formulated to remedy this missing guidance:
2) Contribution to the Development of Guidelines for Toll Plaza Design,” Journal of Transportation Engineering, May/June 2001, McDonald and Stammer.
|Guideline||ETC Dedicated Lane Guideline 1|
|Title||ETC Dedicated Lane Count|
|Text||For new construction, where express lanes are not feasible, the number of dedicated lanes should at least equal the number of approach roadway lanes or the total roadway design volume divided by 1,500.|
|Commentary||This provision is intended to assure a given toll plaza will not require future modifications or lane conversions to meet customer demand for non-stop lanes. In theory, it may be appropriate to defer implementation of dedicated lanes for new toll plaza construction, and rely on mixed-use lanes until ETC participation increases to fully support the use dedicated lanes. In practice, new toll roads rely heavily on non-stop ETC to garner public support and acceptance.|
|Guideline||ETC Dedicated Lane Guideline 2|
|Title||ETC Dedicated Lane Orientation – Mainline Toll Plazas|
|Text||Where possible, lanes accepting the same payment types should be clustered. On mainline plazas, dedicated lanes should be clustered to the left as vehicles approach the plaza.|
|Commentary||Exceptions may be warranted when approach or departure zones are located on interchange ramps, or high volumes of commercial traffic are present. Under these conditions, a supplemental dedicated lane towards the right of the plaza to support traffic entering or exiting the system in multiple directions, or to isolate ETC commercial vehicles away from heavy commuter traffic in the left dedicated lanes, may be warranted.|
|Guideline||ETC Dedicated Lane Guideline 3|
|Title||ETC Dedicated Lane Orientation – Ramp Toll Plazas|
|Text||Assignment of ETC dedicated lanes is dependent on ramp geometry and proximate merges and splits. Consistent locations should be used to enable quick recognition and simplify the plaza approach for repeat drivers.|
|Commentary||The conversion of cash toll collection lanes into dedicated lanes at ramp plazas has been challenging, particularly for ticket system operators which have plazas located close to at least two merges and splits in trumpet interchanges. Dedicated lane selection must be made based on traffic characteristics of the individual toll operator. The New York Thruway, for example, often uses center lanes in these plazas for dedicated lanes, as this allows a single dedicated lane to serve traffic departing the plaza area to the left or the right without weaving.|
|Guideline||ETC Dedicated Lane Guideline 4|
|Title||Directional Separation of Traffic – Mainline Toll Plazas|
|Text||As mainline plazas are upgraded with ETC dedicated lanes, opposing directions of traffic should be separated by permanent barrier, or moveable barrier for reversible lanes, that is capable of absorbing the impact of a vehicle with limited movement and deflection, except where the separation between opposing directions equals or exceeds highway clear zone standards.|
As ETC participation climbs above 50%, the need for reversible toll lanes lessens in most locations other than those with reversible mainline lanes. Permanent barrier is expected to assist the driver in navigating the plaza. Also, the task of moving cones to shift plaza centerlines is a dangerous field assignment, and with increasing driver speeds is becoming more dangerous. Notwithstanding the use of rigidly followed safety procedures when visibility is good, this practice should be discontinued as soon as it is feasible.
For major bridge and tunnel crossings, where significant reversible lane or contra-flow traffic operations are used, the use of moveable concrete barrier could be considered if the expense is warranted.
|Guideline||ETC Dedicated Lane Guideline 5|
|Title||ETC Dedicated Lane Widths|
|Text||Twelve feet (3.6m) is the recommended width for dedicated lanes that allow commercial vehicles (CV). For dedicated lanes that only allow passenger cars, 11 feet (3.4m) is the recommended minimum width.|
|Commentary||Retrofits of existing plazas may deviate from these guidelines, but the designer needs to consider expected operating speed and protection of adjacent obstacles.|
|Guideline||ETC Dedicated Lane Guideline 6|
|Title||ETC Dedicated Lane Island Widths|
|Text||In the absence of any other site conditions or safety requirements, dedicated lane islands should replicate the dimensions of other conventional plaza islands, in accordance with any agency or adopted design standards.|
|Commentary||For new or reconstructed facilities, island width should be dictated by the more controlling toll booth width plus lane clearance or lane clearance plus safe access to toll island equipment.|
|Guideline||ETC Dedicated Lane Guideline 7|
|Title||ETC Dedicated Lane Posted Speeds|
|Text||Given compliance with other dedicated lane guidelines, a maximum-posted speed of 25 - 30 mph (40 – 48 kph) is recommended. In locations with many curves, merges and diverges within several hundred feet (i.e., one hundred meters) of the plaza, lower maximum-posted speeds are recommended.|
|Commentary||Maximum-posted speeds lower than 25 mph may apply for ramp plazas located within trumpet interchanges.|
|Guideline||ETC Dedicated Lane Guideline 8|
|Title||ETC Dedicated Lane Speed Differential Mitigation|
|Text||Barrier or pavement markings are recommended to separate dedicated lanes from cash lanes for a length of approximately one half of the queue zone.|
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