Civil Engineering Reference
In-Depth Information
To address this diamond interchange issue several treatments have been applied
that include (a) building additional crossings over or under the freeway lanes and
(b) recon
guring the arterial roadway, and its ramp connections.
One solution
—
applied along the I-494 freeway in the Detroit metropolitan area
—
600 ft from
the arterial road. These bridges form a one-way couplet to accommodate left
turns from both the freeway exit ramps and the intersecting arterial roadway.
Left turns are prohibited at frontage road intersections with the arterial thereby
simplifying the traf
was building two one-way bridges over the freeway spaced 400
-
c signal phasing.
A similar possibility is to build a new bridge over the freeway and connect it to
the existing street system to form a one-way couplet with the existing bridge
across the freeway.
The most far reaching treatment of diamond interchanges is what is called the
“
displaced diamond
”
interchange -
“
DDI
”
-or
“
double crossover
”
intersection.
This approach is amply documented in the TRB publication
Design and
Operation Performance of the Double Crossover Intersection and Diverging
Diamond Interchange.
“
”
[
12
]. This treatment simpli
es the traf
c signal phasing
but it requires more space along the crossroad.
The interchange designs of new freeways should be tested for their congestion
impacts, and the necessary modifications made to minimize traffic congestion.
17.2.2.6 Separating Grades
Grade separations (
(flyovers and underpasses) have been built at heavily traveled
and complex road junctions in many cities to avoid severe congestion. The higher
volume movement is removed from the at grade intersection thereby resulting in
reduced delay
fl
both on the main arterial and cross street.
Some applications of intersection grade separations are given in Table
17.3
.
—
Most treatments carry the main arterial under or over the intersecting streets
Left-turn
fl
flyovers are found in Chicago and Miami Beach
A
at the intersection of Telegraph Road and Eight mile
Road in metropolitan Detroit separates the high volume through movements on
both six to eight lane roadways; the middle level of the interchange is signalized
to accommodate the turning movements on both roadways
“
three-level diamond
”
The Route 4 and 17 cloverleaf interchange in New Jersey separates grades on
two major roadways with expressway characteristics
Connecticut Avenue in Washington, DC originally had two levels below the
street: there was a short two-track street car tunnel on the upper level that was
closed when street car service was abandoned; and a longer lower level roadway
tunnel.
