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:

1. Left turns shall yield to oncoming traffic.
2. The driver reaching the intersection first has the right-of-way unless turning left.
3. When two vehicles reach the intersection simultaneously, the one on the right has the right-of-way.
4. At uncontrolled intersections, the left turn shall proceed immediately after the oncoming straight ahead driver goes.
5. 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.
6. 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.
7. A YIELD sign means stop IF another vehicle is approaching.
8. 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.
9. 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.
10. Traffic facing a FLASHING CIRCULAR RED may, after a complete stop, cautiously enter the intersection after yielding to all conflicting traffic.
11. 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.
12. Traffic facing a steady burning GREEN ARROW has exclusive right to enter the intersection to make the indicated movement free from conflict.
13. Traffic facing a FLASHING YELLOW ARROW may enter the intersection to make the indicated movement after yielding to oncoming traffic and pedestrians.
14. 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 movement
15. 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 ended.
16. 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.
17. 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 is allowed.
18. 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 one-way.

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:

1. 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.
2. 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 STOP.

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:

1. 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 treatments:
   • 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 well).
   • 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.

2. 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.
3. 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.
4. A left turn detector might not detect cars waiting in the intersection to turn through gaps in traffic on the circular green.
5. 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!).
6. 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.
7. 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.
8. 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 there.
9. 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 apply:

1. To the left turning driver, the flashing yellow arrow and the circular green indications have the same meaning.
2. The difference between the flashing yellow arrow and the circular green is the meaning each indication gives to traffic that is NOT turning left.
3. 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.
4. 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 indications:

DIVERTING TURNS:

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 intersection:

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.

1. 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.
2. 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 original intersection.
3. 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 original intersection.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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 intersection.
9. 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.
10. 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.
11. 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 main intersection.
12. 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.
13. 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.
14. 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 intersection.
15. 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:

1. 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.
2. 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 interchange.
   • 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 interchange.

TURN ON RED PROBLEMS:

There are several problems with turns on red that interfere with left turns. Here they are:

1. 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.
2. 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.
3. 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.
4. 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 street traffic.
   • 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.

QUEUE PROBLEMS:

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:

1. 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.
2. 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.
3. If the left turn lane is too short, traffic waiting to turn can back up into other lanes, blocking them.
4. 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.
5. 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 same street.

Links

• HOME
• How Left Turn Signals Work
• Unusual Traffic Hazards