Approach Zone Design Issues
Some significant issues to consider in designing approach zones include the following:
Approach Zone Guideline Development
The design development work performed in 1999 by McDonald(1) and in 2001 by McDonald and Stammer should be used as the basis for the layout of approach zones. For agencies with existing(2) design standards, the stated reference should function as a supplement.
The number of toll lanes needed to accommodate the forecasted design year traffic volumes can be determined manually by the use of queue theory models and calculations or through the use of plaza simulation techniques. Some examples of simulation packages are the following:
TOLLSIM, is a simulation model developed by Wilbur Smith Associates to analyze the toll operation at the approach to the toll plaza. It requires traffic data and lane type configuration, ramp approaches and the storage length of each lane. The model produces simulation analysis results in graphic and number format, listing a number of measures of effectiveness such as delay per lane, delay overall, and queue length. No analysis of traffic operation downstream from the toll plaza can be performed.
VISSIM, is a general simulation model that can be tailored for toll plaza performance analysis. It requires the same input data listed above under TOLLSIM. In addition, calibration is required to match existing toll operations in the field. The advantage of VISSIM is that the user can analyze the highway leading to the plaza, downstream from the plaza and at the plaza. The interactions between these locations are seamless in this model.
CORSIM, is also a simulation model commonly used for highway corridors. It can be used in conjunction with TOLLSIM to analyze the traffic operations on the ramps and local r oadway system downstream from the toll plaza. Although it can theoretically simulate operations at a toll plaza, this is not a straightforward modeling effort, requiring resolution of some inherent complications
|Guideline||Approach Zones Design Guideline 1|
|Title||Queue Zone Lengths|
|Text||The length of queuing zone should be based on estimated or actual peak hour queue lengths, determined by simulation and analysis, plus an added safety factor, with a minimum of 200 feet. Design year traffic volumes should be used.|
|Commentary||For plaza reconstruction and expansion, design should make use of a simulation model to calibrate existing plaza operations and to estimate plaza queuing and toll lane usage, or use professionally acceptable manual calculation methods (note: vehicle mix, daily/weekend/holiday profiles, and unusual demand generators). The analysis must account for increased usage of express lanes and ETC dedicated lanes, which is expected to reduce conventional plaza queuing in the future.|
|Guideline||Approach Zones Design Guideline 2|
|Title||Transition Zone Tapers|
|Text||Transition zone tapers approaching the conventional plaza should use the minimum taper rates presented in the McDonald 1999 and McDonald and Stammer 2001 reports.|
|Commentary||Reference the ITE Freeway and Interchange Geometric Design Handbook – Chapter 13 for further design information on taper rates.|
|Guideline||Approach Zones Design Guideline 3|
|Title||Proximity to On-ramp|
|Text||If the distance to safely change lanes to access the express lanes after entering the mainline from an upstream interchange on-ramp is not sufficient, this movement should be physically prevented through the use of barrier or delineator separated auxiliary lane extensions or an extended on ramp pavement grade separation beyond the express lane/conventional plaza gore.|
|Commentary||Existence of an ETC dedicated lane or provisions to add one within the conventional plaza should minimize any inconvenience to the ETC customer.|
|Guideline||Approach Zones Design Guideline 4|
|Text||The approach transition zone begins at the start of the gore where the conventional plaza and express lanes split.|
|Commentary||For tunnel plazas, sensors and physical constraints should be deployed at this location to prevent oversized trucks from entering a toll lane. Provisions for safely maneuvering the vehicle out of the approach zone are required.|
Table of Contents | List of Tables | List of Figures | Previous Section | Next Section | HOME