4.0 Geometric and Safety Design

4.2.2 Recommended Guidelines

Departure Zone Design Issues

Some of the significant issues to consider in establishing departure zones include the following:

available right-of-way or space,
current or future merge with express lanes,
proximity to downstream interchange exit,
number of continuing roadway or ramp lanes after departing conventional plaza,
number of conventional plaza toll lanes, both current and future, including ETC dedicated and cash lanes,
relative acceleration rates and speed of vehicles in the recovery zone,
provisions for oversized trucks to turn around or other safe maneuver to exit a tunnel plaza area,
profile grade and cross slopes for low maintenance drainage,
pavement materials to minimize frequency of rehabilitation and reconstruction,
provisions for buses and RV traffic to stop.

Departure Zone Guideline Development

The design development work performed in 1999 by McDonald and in 2001 by McDonald and Stammer should be used as the basis for the layout of departure zones. For agencies with existing design standards, the stated reference should function as a supplement.

Guideline	Departure Zones Design Guideline 1
Title	Recovery Zone Lengths
Text	The departure recovery zone should be equal to at least 200 feet and preferably 300 feet, a length expected to allow vehicles sufficient driver re-orientation, acceleration, and initial merge distance after exiting the plaza.
Commentary	For tunnel toll plazas, a longer recovery zone may be warranted for oversized vehicles to safely maneuver out of the plaza area if sensors and physical constraints are not available or deployed to detect an oversized vehicle prior to entering a toll lane.

Guideline	Departure Zones Design Guideline 2
Title	Transition Zone Tapers
Text	Transition zone tapers departing the toll 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	Departure Zones Design Guideline 3
Title	Proximity to Off-ramp
Text	If the distance to safely change lanes to reach the exit lane of a nearby downstream interchange from an express lane is not sufficient, , this movement should be physically prevented by a downstream extension of the raised median or barrier and gore area separating express lanes and merging conventional plaza lanes.
Commentary	New construction should avoid potential information overload related to informing ETC users to exit the facility through the conventional plaza lanes in lieu of the express lanes by locating the plaza a sufficient distance from entry and exit ramps. For existing facilities, advance signing should be used to direct traffic that will be exiting at an interchange ramp just downstream of the plaza to use the conventional toll plaza lanes in lieu of the express lanes. Existence of an ETC dedicated lane or provisions to add one within the conventional plaza should minimize any inconvenience to ETC customers. The number of ETC dedicated lanes required should be calculated by determining the percentage of ETC users exiting at a nearby downstream interchange ramp after traveling through a conventional plaza and or percentage of ETC users entering the conventional plaza from a nearby upstream interchange on ramp. These percentages are then converted to volumes of ETC traffic.

Guideline	Departure Zones Design Guideline 4
Title	Express Lane Departure
Text	The departure recovery and transition zones should be fully completed (i.e. merge to the number of lanes that equals the roadway typical section downstream of the gore with the continuing express lanes ) prior to the merge with the continuing express lanes.
Commentary	An auxiliary lane may be used to temporarily increase the number of lanes merging with the continuing roadway lanes after exiting the conventional plaza.

Guideline	Departure Zones Design Guideline 5
Title	Recovery Zone Dedicated Lane Design
Text	Placement of physical separation devices for dedicated lane traffic should be extended beyond the toll islands until traffic in the adjacent lanes that had stopped to pay the toll has accelerated to 50% of the design speed. As a minimum, solid white striping should continue until the accelerating traffic has reached a point of approximately two-thirds (66%) of the design speed based on the average acceleration rate of a mid-size vehicle.
Commentary	If an advance barrier is used, breaks in the barrier should not allow “last chance” lane changes. This can disrupt traffic flow and could contribute to accidents.

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