is background information on median acceleration lanes from NCHRP Report 375.
acceleration lanes are increasingly used at intersections on high-speed divided
highways. However, median acceleration lanes are not appropriate for all divided
highway intersections. The 1994 AASHTO Green Book states the following on pages
750 and 751:
lanes are not always desirable at stop controlled intersections where entering
drivers can wait for an opportunity to merge without disrupting through
traffic. Acceleration lanes are advantageous on roads without stop control and
on all high volume roads even with stop control where openings between vehicles
in the peak hour traffic streams are infrequent and short.
medians, where the opposing left-turn lanes do not overlap, median acceleration
lanes can typically be provided within the width used for the conventional
left-turn lane in that same direction of travel. Thus, median acceleration
lanes can be incorporated in many existing designs without widening the median.
However, the presence of a median acceleration lane reduces the amount by which
the conventional left-turn lane for the opposing direction of travel on the
divided highway can be offset. Thus,
providing an operational and safety advantage for left-turns onto the divided
highway may create an operational and safety disadvantage for left-turns from
the divided highways.
A 1986 survey
by the Institute of Transportation Engineers (ITE) found that median
acceleration lanes had been used by 13 of the 53 highway agencies (25 percent)
that responded to the survey. Respondents to the survey were divided evenly for
and against the use of median acceleration lanes. ITE concluded that the median
acceleration lanes appear to promote efficient left-turns onto major roadways
and to reduce accidents and traffic conflicts but that insufficient data are
available to quantify their traffic operational and safety benefits.
On the basis
of the guidelines used by state highway agencies, acceleration lanes for
left-turning vehicles from a crossroad onto the divided highway should be
considered at locations where adequate median width is available and the
following are true:
Limited gaps are available in the
major-road traffic stream.
Turning traffic must merge with
high-speed through traffic.
There is a significant history of
rear-end or sideswipe accidents.
ISD is inadequate.
There are high volumes of trucks
entering the divided highway.
agency guideline stated that a truck volume of 75 to 100 trucks per day would
be sufficient to warrant a median acceleration lane.
Median acceleration lanes, like offset left-turn
lanes, have come into use at divided highway intersections in recent years but
have been used by only a few agencies. A typical median acceleration lane is
shown in Figure 9. The advantage of median acceleration lanes is that they
allow vehicles turning left onto a divided highway to continue through the
median roadway without stopping and merge onto the far roadway of the divided
highway. That allows drivers to cross
the near roadway of the divided highway without considering the availability of
gaps on the far roadway, even if the median is not wide enough to store their
vehicles; however, even if a median acceleration lane is provided, left-turning
drivers must anticipate potential conflicts with other vehicles in the median
area. Thus, the presence of a median acceleration lane changes the decision
making process and the maneuvers made by the crossroad drivers turning left onto
the divided highway, but not by drivers crossing the divided highway. The
median acceleration lane should be long enough to allow left-turning vehicles
to reach the speed of major-road traffic before merging.