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Basic Freeway Section and Ideal Freeway Conditions
A basic freeway section is a segment where there are no
interruptions to the flow of traffic. Interruptions to trafficflow
occur when vehicles enter or leave the freeway. Therefore, a basic
freeway section is one where on or off rampsare not present for at
least 1500 feet upstream and downstream of the section.
In addition to uninterrupted conditions, the "ideal" basic
freeway section is defined as having the
followingcharacteristics:
o Each lane is 12 feet wide.o There is 6 feet of clearance
between the outside and the inside edges of the freeway and the
nearest
obstruction that would distract or influence a motorist.
o All vehicles are passenger cars (no trucks, buses, or
recreational vehicles).o Ten or more lanes (in urban areas only).o
Interchanges are spaced every 2 miles or more.o The drivers are
regular and familiar users of the freeway section.o The terrain is
level, with grades no greater than 2%.
Together, these conditions represent the "highest" (ideal) type
of freeway section, which is one with a free-flow speed
of 70 mph or higher and a capacity of 2400 passenger cars per
hour per lane (pcphpl).
Basic Freeway Section and Ideal Freeway Conditions
The following excerpt was taken from Chapter 3, page 3-1, of the
1997 revision of the Highway CapacityManual published by the
Transportation Research Board.
A freeway may be defined as a divided highway with full control
of access and two or more lanes for the exclusiveuse of traffic in
each direction.
Freeways provide uninterrupted flow. There are no signalized or
stop-controlled at-grade intersections, and directaccess to and
from adjacent property is not permitted. Access to and from the
freeway is limited to ramp locations.
Opposing directions of flow are continuously separated by a
raised barrier, an at-grade median, or a raised trafficisland.
Operating conditions on a freeway primarily result from
interactions among vehicles and drivers in the traffic streamand
between vehicles and their drivers and the geometric
characteristics of the freeway. Operations can also beaffected by
environmental conditions, such as weather or lighting conditions,
by pavement conditions, and by theoccurrence of traffic
incidents.
Freeway vs Highway
All freeways are highways but not every highway is a freeway. A
freeway is a "controlled-access" highway that's
designed exclusively for high-speed vehicular traffic. Traffic
flow on a freeway is unhindered because there are notraffic
signals, intersections or anyat-grade crossingswith other roads,
railways, or pedestrian paths.
The main difference between freeways and multilane highways is
that in the case of freeways, these roads are
separated from the rest of the traffic and can only be accessed
by ramps. These ramps (slip roads) allow for speed
changes between the freeway and arterial thoroughfares and
collector roads. Opposing directions of traffic on a
freeway are physically separated by a central reservation
(median), such as a strip of grass or boulders, or by a traffic
barrier. Traffic across a freeway is carried by overpasses and
underpasses.
http://en.wikipedia.org/wiki/At-grade_intersectionhttp://en.wikipedia.org/wiki/At-grade_intersectionhttp://en.wikipedia.org/wiki/At-grade_intersectionhttp://en.wikipedia.org/wiki/At-grade_intersection
