Parkways and park roads | ||
GRADE SEPARATIONS AND INTERCHANGES
from Chapter X, A Policy on Geometric Design of Highways and Streets,
NCHRP, December 1979
"The greatest efficiency, safety, and capacity are attained when
intersecting through-traffic laws are separated in grades." "The
type of grade separation and interchange along with their design,
is influenced by many factors, the principle factor being design designation....traffic volume, character of composition of traffic,
design speed, and type of control of access."
Other controls: signing, economics, terrain, right-of-way.
Basic type of interchanges can vary extensively in shape and scope.
Fig. X-1A-Trumpet or jug-handle ramp configuration
Fig. X-1B-Three-level, directional, three-leg interchange
Fig. X-1C-Not suitable for freeways but practical for highway-parkway
connections where trucks are prohibited and
design speed is lowFig. X-1D-Typical diamond which has variations with frontage
road and collector-distributor rampsFig. X-1E-Partial cloverleaf which favors heavier traffic volumes
Fig. X-1F-Full cloverleaf generates weaving movements that must
occur on collector-distributor roadsFig. X-1G-Fully directional interchange-example is four-stack
interchange in Los Angeles
Open-road capacities can flow without interruption when intersecting
roads are separated by a structure. The high initial cost
of grade separations must be justified on the two considerations of
(1) elimination of traffic bottlenecks, and (2) correction of existing
hazards. Six items (or warrants) will justify an interchange:
Design Designation: whether or not the access will be fully
controlled between terminalsElimination of bottlenecks or spot congestion: inability to
provide essential capacity of one or both roadsElimination of hazard: locations of high accident frequency
Site Topography: physical properties of site make at-grade
intersection impossibleRoad-User Benefit: costs due to delays, fuel, time and
accidents require improvement; relation of road-user benefit
to cost of improvement; annual benefit divided by annual2
cost of improvement; annual cost is sum of interest plus
annual amortization.Traffic Volume: volumes exceed capacity of at-grade intersection;
elimination of conflicts greatly improves movement
of traffic.
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6.
Additional Warrants:
Local streets cannot be terminated outside of right-of-way
Access to areas not served by frontage roads or other access
Railroad grade separations
Unusual concentrations of pedestrians such as parks on both
sides of roadwayBikeways or pedestrian crossings
Access to mass transit within major arterial
Free-flow aspects of certain ramps and completing geometry
of interchange
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Parkways and park roads | ||