4.0 Geometric and Safety Design

4.6.2 Recommended Guidelines

Cross Slope Design Issues and Guideline Development

Some of the significant issues to consider when designing a toll island and recommendations for resolving these issues include the following:

slopes that promote storm water runoff from roadway and canopy,
location of inlets, grates, catch basins and other drainage facilities to handle runoff,
consideration of superelevation for conventional plazas constructed on roadway curves,
use of cross slopes and profile grades in lieu of inlets within the toll lanes to prevent ponding and assure heavy drainage flows have no affect on safety and throughput, particularly as a result of a clogged drain,
use of minimal cross slope in lanes where treadles are installed to avoid any degradation in performance,
rollover of adjacent lanes with differing cross slopes.

Cross Slope Design Guidelines

Guideline	Cross Slope Design Guideline 1
Title	Cross Slope Ranges
Text	Design of cross slopes should follow the methods and guidelines provided by the AASHTO “Green Book” and or the respective state DOT design manuals. Drainage design needs to address runoff from the canopy. Cross slopes will typically range from 1%-2% on tangent roadway sections and higher on curves.
Commentary	The drainage design must combine cross slopes with longitudinal grade to avoid any water ponding within the toll plaza. Canopy runoff should be directly piped into the drainage system.

Guideline	Cross Slope Design Guideline 2
Title	Rollover Ranges
Text	Design of rollover of adjacent lanes should follow the methods and guidelines provided by the AASHTO “Green Book” and or the respective state DOT design manual. Rollover should be limited to a combine grade differential of 4%.
Commentary	None

Federal Highway Administration - United States Department of Transportation