10.6.1 Overview
See
on possibility of converting a 2-lane undivided facility to a Super 2 or other configuration.
A Super 2 highway is one in which periodic passing lanes are added to a two-lane rural highway to allow slower vehicles to pass and traffic platoons to disperse. Passing lanes are provided periodically in each direction of travel along the Super 2 corridor, allowing passing opportunities in both directions. A Super 2 project can be introduced on an existing two-lane roadway where there is a significant amount of slow-moving traffic, limited sight distance for passing, prevalence of head-on crashes, and/or the existing traffic volume has exceeded the two-lane highway capacity (creating the need for vehicles to pass on a more frequent basis).
Widening of the existing pavement can be symmetric about the centerline or on one side of the roadway depending on ROW availability and ease of construction.
shows nine different configurations of passing lanes. The isolated passing lane shown in
a is typically used to reduce delays occurring at a specific isolated bottleneck and is not truly a Super 2 corridor treatment. The other configurations allow some interaction between consecutive passing lanes in opposite directions, and they are used when traffic improvements are needed in both directions of travel over a corridor. The alternating passing lanes shown in
and
can be used when sufficient width is available;
is the typical cross-section for what is commonly described as a 2+1 road. Overlapping passing lanes, shown in
and
, can be used when a passing lane is located on a crest or sag vertical curve, respectively. Side-by-side passing lanes, shown in
, can be used where the location of a passing lane is constrained by nonflexible factors. Those factors include (but are not limited to) obtaining ROW, when heavy traffic is the cause of platooning rather than no-passing zones, and where the need for passing lanes exists in both directions
Because the Super 2 design allows such a high degree of flexibility, the practitioner has a great deal of latitude in the details of the design and implementation of a Super 2 corridor. This flexibility also means that the practitioner has a responsibility to exercise good engineering judgment in determining which details to include when designing and constructing the passing lanes and any other elements associated with a project that contains a Super 2 component. Many of the design details are consistent with the principles used for any rural highway, but the context of the passing maneuver on an otherwise two-lane highway needs to be considered.
contains information on comparing cross-section alternatives and how to determine which of those alternatives to select for a given location based on operational, safety, and economic measures.
Figure 10-1: Passing Lane Configurations
Source: Mutabazi et al. (1999)
Some issues to consider when designing a Super 2 project:
- Analyze existing ROW width considerations to determine feasibility of upgrading to a Super 2;
- Consider providing a left turn or right turn lane if a significant traffic generator falls within the limits of a Super 2;
- Consider providing wider shoulders (8-ft to 10-ft) in areas with high driveway density;
- Evaluate the location and associated treatment to achieve clear zone values at large drainage structures and bridges when considering the placement of passing lanes;
- Evaluate traffic operations including truck volumes if consideration is given to terminating passing lanes on significant uphill grades. Coordinate passing lanes with climbing lane needs to improve operating characteristics;
- Avoid terminating a passing lane over a hill or around a horizontal curve where the pavement surface at the end of the taper isn't visible from the beginning of the taper;
- Provide adequate sight distance (stopping sight distance (SSD) preferable) between the end of a lane termination taper and a constraint such as metal beam guard fence, a narrow structure, or major traffic generator; and
- Consider providing the passing lane in the direction leaving an incorporated area for potential platoons generated in the urban area.
Where passing lanes are provided in the vicinity of intersections, appropriate separation between the extents of passing lanes and turning lanes should be provided based on the guidance from
. The beginning or end of a passing lane should be at least 1000 ft upstream of an intersection and at least 1500 ft downstream of an intersection to avoid introducing intersection delay associated with the interactions of the minor-road vehicles turning on the major road and those vehicles merging at the end of the passing lane. For guidance from
also includes the consideration of appropriate sight distance at the intersection, refer to
.