Lead and Lag left signals
Monte Castleman
many years. Now I am seeing more and more where one direction has a lead and
one has a lag. Usually these are suburban intersections with high traffic
and turning volumes and are always protected only. What's the safety or
operational advantage to this? Obviously there must be one or they won't
waste time reprogramming lights.
John Lansford
IIRC lag lefts work best when there's a big difference in volumes in
the two directions on the main road. The signal can be set up so the
heavier traffic volume has more left turn time without stopping
traffic in that direction, giving the through traffic more time
overall.
John Lansford, PE
--
John's Shop of Wood
http://wood.jlansford.net/
James Dunlop
When intersections are spaced unevenly, using lag instead of lead left
turn phases can help progress the main street flow by allowing the
platoon to flow smoothly in both directions. Traffic engineers use
what's called a "time-space diagram" to figure out the best coordination
for a corridor. The idea is to keep platoons of vehicles moving,
without repeated stopping and starting.
The idea road for this is a one-way street, as the signals can be set to
turn green as the lead vehicles in a platoon arrive at the design speed
of the system, usually the speed limit. Driving in Manhattan, for
instance, you can see the signal progression by watching the signals up
or down one of the one-way avenues. (Often, the signal turns green a
little before the platoon arrives, to get vehicles that turned onto the
main street from other roads a chance to get started; they then become
the leading vehicles of the platoon.)
In the case of diamond interchanges, the lead/lag allows progression of
ramp traffic, especially if the distance between the two ramp
intersections isn't enough to store the turning vehicles, and traffic
could back up and block the ramps (causing gridlock.)
A quick google search found this good description of signal coordination
and time-space diagram...
H.B. Elkins
There is a safety value as well as a congestion mitigation value in certain
spots.
The traffic light at US 441 (Parkway) and Airport Road in Gatlinburg, TN has had
an advance green for southbound traffic for years. The reason is that Parkway
has only four lanes and a LOT of traffic turns left onto Airport road, using the
leftmost southbound lane as the turning lane. Giving them an advance green while
the northbound traffic is held by the red light allows for safe turns and cuts
down on the potential for wrecks as well as clearing out turn traffic in the
left lane. It's been about 9 years since I've been down there but there used to
be "Advance Green" and "Delayed Green" signs in the appropriate spots on 441 to
inform drivers.
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Steve Riner
wrote:
I never appreciated lagging protected left turns until I visited
Tucson, where this phasing is commonly used (along with permissive
turns during the thru green cycle). My observation was that, instead
of traffic moving into the intersection to turn and then taking
several seconds to clear after the light turned red, thus delaying the
cross street's movements (and risking colliding with oncoming traffic
blowing the red light), with the lagging protected left turn it
resulted in no residual traffic in the intersection.
Obviously, considerations would be different if the turns were
protected-only.
Steve Riner
Pueblo West CO
Explore New Mexico and Minnesota highways: http://www.steve-riner.com
PatOConnell
> On Sun, 11 May 2008 14:46:21 -0700 (PDT), Steve Riner
> <highplain...@msn.com> wrote:
>
>> On May 10, 7:19 pm, "Monte Castleman" <mdcastle...@earthlink.net>
>> wrote:
>>> Except at diamond interchanges Minnesota has used lead lefts exclusively for
>>> many years. Now I am seeing more and more where one direction has a lead and
>>> one has a lag. Usually these are suburban intersections with high traffic
>>> and turning volumes and are always protected only. What's the safety or
>>> operational advantage to this? Obviously there must be one or they won't
>>> waste time reprogramming lights.
>> I never appreciated lagging protected left turns until I visited
>> Tucson, where this phasing is commonly used (along with permissive
>> turns during the thru green cycle).
>
> that I actually appreciate. :)
Albuquerque has both leading and lagging protected left turns. Most of
the lagging turns are at freeway entrances, but not all freeway
entrances have lagging turns. In other words, watch the light, and don't
try to anticipate the turn signal, 'cause you might be wrong.
Justin Rhodes
For the most part I like to think that if left turn volumes are enough
to justify a two lane left turn bay, then the signal should be
protected... But, I do think Tucson gets it right with its setup. For
one, you don't have to wait for everyone to turn first before you can
go straight. The way Tucson is set up, with basically three major
surface roads going across town, a leading/protective setup would
probaly cripple east/west traffic.
Also, what I tend to see up here in Phoenix is, with the limited
window of opportunity to turn left that a leading/protective signal
provides, people often drive up to the intersection way too fast and
make a high speed left turn, or they go normal speed and get flashed
by the camera if they enter the intersection after the arrow turns
red.
The setup in Tucson pretty much eliminates that problem, since the
window of opportunity to turn left is a lot larger (traffic
permitting), and the arrow comes on at the end, so there's not as much
rush to get there.
C.O.
turns. Mostly in Scottsdale, Gilbert and Goodyear. Other areas of the
valley have a few lagging lefts scattered around, mostly in areas
where multiple signals are close together. What I have seen more of is
a leading/lagging left, where one direction gets a leading left turn,
and the other side gets a lagging left turn. This is most common at
signals near where the main road crosses a freeway.
The permissive dual left turn intersections are seen mostly in the
Tucson city limits. I have seen a few permissive dual lefts in the
Phoenix area, but that's the exception and not the rule. Areas of
unincorporated Pima County, Marana and Oro Valley tend to do their
signals like the rest of Arizona. I noticed that the last time I was
in Tucson there were more protected left signals in the city limits
than I remember from when I lived there.
Justin Rhodes
According to the City of Scottsdale website, lead lefts are better
when the city has a large grid. In scottsdale, there are three major
north/south roads, so the lagging left works better Same for Tucson
with its east/west traffic. Whereas in Phoenix, the major roads are
mostly a large grid, so lead lefts are better.
You are correct about permissive/protective dual lefts being the
exception in Phoenix, so when I see them I usually take note. There's
one at at 7th ave and Jefferson, another on Roosevelt at 7th Street,
and one in Scottsdale at 90th Street and Shea. There used to be a
permissive/protective dual left at Chandler Blvd and Loop 101, but it
has since been changed to protective...
There's an unsignalized dual left in Phoenix at 27th ave and
jefferson.
Then of course we have several SPUIs up here in phoenix with
unsignalized dual rights. Those can be pretty hair-raising.
Another interesting right turn setup I've seen is at the 51 and Shea.
The outside right turn lane gets channelized into an added lane on
Shea, while the inside right turn lane yields to traffic. According to
the diagram of the new interchange at I-17 and carefree highway, it
will have a similar setup for northbound-to-eastbound traffic.