Civil Engineering Reference
In-Depth Information
Table 16.5 Treatment of left turns at intersections and driveways
Option
Condition
Application considerations
Prohibit
Full time
Requires alternate routes
Peak periods
only
Requires alternate routes
Provide
Share lane
Limit to minor roads or places where R/W is not available for
left-turn lane
Left-turn lane
Protected or permissive phasing
Dual left-turn
lane
Protected phasing only
Divert
Jug-handle
Divided highways at minor roads (signalized junctions only)
Modified jug-
handle
6-lane divided highways
Michigan
“
U
”
Divided highways with wide median
—
allows two-phase
signals
Separate
Directional
design
Very heavy turns in one direction
Left-turn
fl
yover
Very heavy turns in one direction
Through lane
flyover
Major congestion points
Source
Reference [
5
], p 70. Table 8.1
prohibition simpli
es traf
c signal phasing, reduces queues, and improves
ow.
(b) Allowing Left Turns at Intersections
Allowing left turns from a lanes used by through traf
traf
c
fl
c (shared lanes), should
be avoided along major roadways. Left turns from shared lanes can reduce
lane capacity by about 50 %, increase delay to through vehicles, and increase
the number of crashes. For these reasons,
protected
left turn lanes should be
provided where space permits.
Sometimes left turn lanes can be provided by adjusting the center median, or
by reducing the width of through lanes. Providing
five 10-foot lanes on a 50
foot road out of four 12.5-foot lanes, would enable the extra lane to be used as
a protected left turn lane.
Where left turn lanes are provided at signalized intersections, they are
sometimes given a special phase. However, the green time provided for the left
turns can both increases the cycle length and the delays to opposing through
traf
c.
Two-way left turn lanes are common in suburban settings where one
nds
roads with frequent driveways serving roadside development. Left turn lanes
remove left turning vehicles from the through lanes and reduce delays to
through traf
c. Conventional left turn lanes are usually provided at signalized
intersections in suburban areas.
