LEFT TURN TRAFFIC RULES
Why they are as they are
Why have a page on left turns? Because they are the most misunderstood part of
traffic law. Many people think that left turns should have the same priority at all
intersections. Here are the reasons there are differences:
THE GENERAL LEFT TURN LAWS:
- Left turns shall yield to oncoming traffic.
- The driver reaching the intersection first has the right-of-way unless turning left.
- When two vehicles reach the intersection simultaneously, the one on the right
has the right-of-way.
- At uncontrolled intersections, the left turn shall proceed immediately after
the oncoming straight ahead driver goes.
- At STOP signs, the left turn shall proceed immediately after the oncoming
straight ahead driver goes, unless traffic that does not have to stop prevents this.
- A driver entering a street, road, or highway from a private driveway shall stop,
just as if a STOP sign were placed at the end of the driveway. Any other traffic
controls placed on the driveway by the traffic authority supersede this rule.
- A YIELD sign means stop IF another vehicle is approaching.
- Traffic facing a steady burning CIRCULAR GREEN may enter the intersection.
Turning traffic must yield to pedestrians in adjacent crosswalks. Left turning
traffic must yield to oncoming traffic, unless given a left pointing GREEN ARROW.
- Traffic facing a FLASHING CIRCULAR YELLOW may cautiously enter the intersection.
Turning traffic must yield to pedestrians in adjacent crosswalks. Left turning
traffic must yield to oncoming traffic.
- Traffic facing a FLASHING CIRCULAR RED may, after a complete stop, cautiously enter
the intersection after yielding to all conflicting traffic.
- Traffic facing a steady burning CIRCULAR YELLOW is advised that the right-of-way
is ending. Vehicles must be out of the intersection before the CIRCULAR RED appears.
- Traffic facing a steady burning GREEN ARROW has exclusive right to enter the
intersection to make the indicated movement free from conflict.
- Traffic facing a FLASHING YELLOW ARROW may enter the intersection to make the
indicated movement after yielding to oncoming traffic and pedestrians.
- Traffic facing a FLASHING RED ARROW may, after a complete stop and yielding to
oncoming traffic and pedestrians, enter the intersection to make the indicated
- Traffic facing a steady burning YELLOW ARROW is advised that the movement
controlled by the GREEN ARROW, FLASHING YELLOW ARROW, or FLASHING RED ARROW has
- Traffic facing a steady burning CIRCULAR RED must stop. It then must not enter the
intersection unless making a turn on red that is allowed.
- Traffic facing a steady burning RED ARROW must stop. It then must not enter the
intersection to make the indicated movement. Under new federal rules, no turn on red
- In some states where right turn on red is generally permitted after a stop, a left
turn may be made on red after a stop, but ONLY IF all legs of the intersection are
The above list contains the rules in use. The following lists will show why the rules
are as they are. The lists are divided into the kinds of intersections used:
UNCONTROLLED AND STOP INTERSECTIONS:
There are two very important reasons why the left turn waits on oncoming traffic.
Here they are:
- On a through street, much less traffic could pass through if each vehicle had
to wait for a left turn before proceeding. It could become as congested as a street
full of ALL-WAY STOP intersections.
- It takes much less time for a left turn to occur after an oncoming straight ahead
movement than it takes if the left turn went first. When the left turn goes first, the
oncoming car has to wait for the left turning vehicle to completely leave the intersection.
If the oncoming car goes first, the left turning driver can start as soon as the oncoming
car has entered the intersection. It then tucks in behind the oncoming car, taking only
half the intersection time it would otherwise take. The two cars share
some intersection time this way. This especially increases the efficiency of an ALL-WAY
TRAFFIC SIGNALS WITHOUT TURN ARROWS:
Where straight ahead traffic has priority, much more traffic can pass through the
intersection on a green light. If left turns had equal priority, much less traffic could
pass through, because each vehicle might have to stop and wait for a left turn before
proceeding. It negates the advantage of a traffic light, and could become as congested
as an ALL-WAY STOP intersection. Left turns still have a chance to turn as traffic thins out
at the end of the green.
TRAFFIC SIGNALS WITH TURN ARROWS:
If it is more efficient for left turns to go last (lag) normally, why do most traffic
signals with turn arrows let the left turns go first (lead), then let the oncoming straight
ahead traffic go afterwards? There are several reasons:
- The rule that left turns normally wait on straight ahead traffic creates a hazard if
one stream of traffic is cut off early so the other one can have a lagging turn arrow. It
is called a Yellow Trap because it happens on lagging left
turn arrows when the oncoming green turns yellow. When the side cut off receives a yellow,
any left turning drivers there think that the oncoming traffic has a yellow too. They
therefore turn in front of live traffic that still has a green light. They do this,
because they must be out of the intersection by the time the red light shows. This
combination causes many accidents. To prevent the Yellow-Trap, any traffic oncoming to a
lagging turn arrow must have at least one of the following
- The oncoming left turn must not exist.
- The oncoming left turn must be prohibited.
- The oncoming left turn must be diverted away from the intersection (see below).
- There must be an ONCOMING TRAFFIC MAY HAVE EXTENDED GREEN sign (This does not work very
- The oncoming left turn must have a turn arrow, and be prohibited from turning on
the circular green.
- Both approaches on the same street must get left turn arrows at exactly the same time.
- The Flashing Yellow Arrows display is correctly used
on all approaches that could be trapped.
Note that phase skip can cause yellow trap with a leading left turn.
- Left turn phases can be skipped if no traffic is there to use them. This is harder
to predict with lagging turns. With leading turns, the signal can make the decision at
the beginning of the time for that street.
- Left turn phases on the same street can be split, with one green arrow on longer than the
other. Again, it is harder to do this with lagging turns.
- A left turn detector might not detect cars waiting in the intersection to turn
through gaps in traffic on the circular green.
- With an actuated left turn phase, it is much easier to decide when to end the leading
left turn movement (Hey! I'm out of cars!) than it is to figure out when to start the
lagging left turn (Now let's see... I got six left turning cars on the east leg, and two
on the west leg. I'll start the east leg 8 seconds earlier. Done! ... OH NO! Here come
ten more on the west leg!).
- It is more efficient to have the light on straight ahead green than turn arrow if
one set of left-turn-vs-oncoming movements runs out of cars earlier than the other. More
stragglers can get through the intersection that way. With leading turn arrows, straight
ahead and right turn stragglers can go on the circular green at the end of the time for
that street. With a lagging turn, only left turn stragglers can go.
- A signal with only leading left turn arrows can allow permissive turns through gaps
in oncoming traffic during the circular green. A signal with a lagging turn arrow must
display a red to oncoming left turning drivers during the oncoming circular green to
prevent yellow trap, unless:
- The signal is at a "Tee" intersection, where there is no oncoming left turn.
- The cross street is one-way, so there is no oncoming left turn.
- The left turn phases are simultaneous, not split.
- The two legs of the street are split, so each leg has its own separate green phase.
- The Flashing Yellow Arrows display is used on
all approaches that could be trapped.
- More sophisticated detection equipment is needed to properly time the lagging turn.
This is especially true if the turns are split, or the signal must change to the cross
street at a certain time to progress cars from one intersection to the next. Many existing
lag turns are actuated with a fixed green arrow interval for this reason. The equipment
must otherwise know HOW MANY cars are waiting, rather than just if cars are still
- How Left Turn Signals Work
TRAFFIC SIGNALS WITH FLASHING YELLOW TURN ARROWS:
The flashing yellow arrow signal is used to prevent yellow trap. The following facts
- To the left turning driver, the flashing yellow arrow and the circular green indications
have the same meaning.
- The difference between the flashing yellow arrow and the circular green is the meaning
each indication gives to traffic that is NOT turning left.
- Where yellow trap occurs on an approach without a left turn signal, a special flashing
yellow arrow signal face with no green arrow must be used.
- Driver understanding is not the reason the flashing yellow arrow was developed.
The flashing yellow arrow left turn signal face tells you the color of the oncoming circular
|LEFT TURN INDICATION
||POSSIBLE ONCOMING INDICATIONS
|Steady Red Arrow
||Stop and stay
||Any steady circular indication
|Flashing Red Arrow
||Stop, and turn when safe
||Steady Circular Green or Flashing Circular Yellow
|Steady Yellow Arrow
||Prepare to stop
||Steady Circular Yellow or Steady Circular Red
|Flashing Yellow Arrow
||Yield to conflicting traffic
||Steady Circular Green
|Steady Green Arrow
||Go - protected turn
||Steady Circular Red
A left turn problem can be reduced or eliminated by engineering one of these solutions.
They move the conflict between left turn and oncoming vehicles away from the
The letters in parentheses refer to the diagrams at right.
The main version has traffic coming from the bottom and leaving at the left.
The existing streets version has traffic entering at the top and exiting at the right.
The side road version has traffic entering from the right.
- One-Way Streets: This moves the oncoming cars to an adjacent parallel street. By
the time the left turning vehicles get there, they are part of the cross street traffic
and go across on the cross street green.
Near Side Jughandle (A): This is a diagonal roadway that branches off to the right before
the intersection. Left turning traffic turns right onto the jughandle road, and then turns
left at a separate intersection on the cross street. The left turn is prohibited at the
- Existing Street Near Side Jughandle (A streets): This version of the near side
jughandle can be improvised using existing streets. The left turn route is simply signed
as a right turn, followed by two left turns. Again, the left turn is prohibited at the
- Far Side Left Jughandle (B): This is a diagonal roadway that turns off to the left
after the intersection. Left turning traffic turns left onto the jughandle road after
passing straight through the main intersection, and then merges with cross street traffic.
The left turn is prohibited at the original intersection. This can use a half signal for
the first left turn, facilitating progression.
- Existing Street Far Side Left Jughandle (B streets): This version of the far side left
jughandle can be improvised using existing streets. The left turn route is signed as
straight through the intersection, then two left turns, followed by a right turn. Again, the
left turn at the original intersection is prohibited.
- Cloverleaf Far Side Right Jughandle (C): This is a loop roadway that branches off to
the right after the main intersection. Left turning traffic goes straight through the
intersection and turns right onto the loop road. It curves around to the right and merges
with the cross street. The traffic then goes straight through the intersection on the cross
street. The left turn itself is prohibited at the intersection.
- Existing Street Cloverleaf Far Side Right Jughandle (C streets): This version of the
cloverleaf jughandle can be improvised using existing streets. The left turn route is signed
as going straight through the intersection, and then making three right turns. Again, the left
turn at the original intersection is prohibited.
- Michigan Left (D, main road): A site to make a U-turn is provided beyond the intersection.
The left turning driver drives straight through the main intersection, then makes a U-turn,
and then turns right at the main intersection. Left turns are prohibited at the main
- Michigan Left (D, side road): A site to make a U-turn is provided to the right of the
intersection. The left turning driver turns right at the main intersection, and then makes a
U-turn. Left turns are prohibited at the main intersection.
- Superstreet (E, side road left): A site to make a U-turn is provided to the right of the
intersection. The left turning driver turns right at the main intersection, and then makes a
U-turn. Side street left turns are prohibited at the main intersection. The superstreet
intersection uses two or four half signals.
- Superstreet (E, side road straight): A site to make a U-turn is provided to the right
of the intersection. The straight ahead driver turns right at the main intersection, makes
a U-turn, and then turns right onto the side road. Straight ahead is prohibited at the
- Bow Tie Left (main road): A roundabout is provided to the right of the intersection.
The left turning driver turns right at the main intersection, and then makes a U-turn in the
roundabout. Left turns are prohibited at the main intersection.
- Bow Tie Left (side road): A roundabout is provided beyond the intersection. The left
turning driver goes straight through the main intersection, makes a U-turn in the
roundabout, and then turns right at the main intersection. Left turns are prohibited at
the main intersection.
- Continuous Flow Left and Parallel Flow Left (below): The left turn is made before the
main intersection, and follows a special road past the main intersection. Then another
left turn is made to enter the other street. Left turns are prohibited at the main
- Free flow methods can be used to eliminate crossing
movements, making turns much easier to do.
CONTRAFLOW LEFT AND DIVERGING DIAMOND INTERCHANGES:
An interchange left turn problem can be reduced or eliminated by engineering one of
these solutions. They move conflicts away from the usual locations:
- Contraflow Left Diamond Interchange (above): Each left turn from each direction on the
crossroad passes to the left of the left turn lane going the opposite direction. Thus,
those left turns do not cross each other's paths. Thus, storage for left turns entering
the ramps is not limited to the spacing between the two ramp intersections.
Diverging Diamond Interchange (right): The two directions of travel on the crossroad cross
each other at traffic lights at the ends of the interchange.
- The left turn from the crossroad crosses to the left side of the road when entering
the interchange, and then has a free left turn onto the ramp.
- The straight ahead on the crossroad crosses to the left side of the road when
entering the interchange, and crosses back to the right side of the road when leaving
- The left turn from the off ramp has a free left turn onto the left side of the
crossroad, and then crosses to the right side of the road when leaving the
TURN ON RED PROBLEMS:
There are several problems with turns on red that interfere with left turns. Here
- Turns on red must be prohibited at any intersection with turn arrows. Failure to
do so undermines the meaning of a turn phase without conflict.
- Many drivers turning on red look at only cars coming from their left. They fail
to notice left turning drivers coming from the oncoming stream. They also fail to
notice pedestrians approaching from the right. Some are so intent on turning right
on red that they fail to notice that their own signal has turned green.
- Many drivers do not understand that if any leg of an intersection is two-way, left
turns may not be made on a red light.
- Turns on red can cause accidents without involving the driver who turned on red:
- The next driver in line might think the signal turned green and collide with cross
- Any driver farther back in line might think the signal turned green and have a
rear-end collision with the next car ahead in the line.
- The driver turning on red might cut off another vehicle that has a green signal,
forcing it into other traffic.
Drivers stuck behind a vehicle waiting to turn left can cause other problems. They want
to get around that roadblock ahead of them, and some go to extreme lengths to do so:
- Some drive off the road to get around, and many get stuck trying to do so. It is
illegal to drive off the road to pass a left turning driver. That's why passing blisters
are installed at many intersections or across from major driveways on two lane roads.
- On a four lane highway without left turn lanes, drivers crowd into the outside lane
to avoid a left turning driver stopped in the left lane. Many close calls and some
accidents result from this.
- If the left turn lane is too short, traffic waiting to turn can back up into other
lanes, blocking them.
- On a high speed road, a car slowing down to turn left can cause sideswipe and rear-end
accidents, if there is no deceleration left turn lane. Vehicles following too close
compound this problem.
- Left turns can take up so much time at multiphase signals (especially those with
quad left turn signals) that traffic backs up in all four directions. Some drivers even
drive on the shoulders or through parking lots to get away from this monster. When this
happens, the only corrective solutions are:
- Double lane left turns (must have a turn arrow).
- One way streets.
- Diverted turns.
- An interchange.
Left turns are different, because they interfere with other traffic on the same street.
Other movements interfere with traffic on the other street, but not with traffic on the