FIXING A BAD INTERSECTION

APPLYING THE FLASHING YELLOW ARROW DISPLAY TO A PROBLEMATIC INTERSECTION THAT CANNOT BE EASILY WIDENED

THE INTERSECTION AS IT IS:

DESCRIPTION

• Walnut Street is a 4-lane undivided north-south street.
• Grimes Lane is a 2-lane undivided east-west street.
• Each of the four approaches has one added left-turn lane.
• The traffic signal has pedestrian signals and a "doghouse" 5-section left-turn signal for each approach
• The signal sequence used is the standard quad lead signal sequence. See the diagram below the photo.
• Because the signal is in a coordinated system, the left turn phases are actuated, but the thru phases are not.
• The heaviest flows of traffic make the following movements:
  1. Northbound straight ahead
  2. Southbound straight ahead
  3. Northbound left turn
  4. Eastbound right turn
• The eastbound and westbound legs are offset. Traffic must move over one lane within the intersection.

• The most obvious problem in the photo is the line of backed up cars in the northbound left-turn lane. It spills out into the left hand lane for straight-ahead traffic, blocking that lane.
• The next problem is the traffic backed up on the eastbound approach. Most of this traffic wants to turn right. Right turn on red takes care of some of the traffic, until a car waiting to go straight ahead must wait for the green light.
• The intersection and its approaches cannot be widened without losing at least one business. If a business has no parking spaces, it can't exist. And at least two buildings would have to be torn down.
• An overpass crosses the west leg not far from the intersection. There is not room to add another lane between the uprights of that overpass.
• Yellow trap is not a problem because the thru phases are never skipped and the left turns always lead.
• Because the quad lead signal sequence lets the left turns go before the straight-ahead traffic goes, the left-turn and straight-ahead vehicles block each other in the left northbound straight-ahead lane. The signal sequence (with some phases skipped on alternate cycles) is shown to the right.
• The use of the "doghouse" left turn signals prevents the use of most other signal sequences that would move the northbound left-turn and straight-ahead vehicles at the same time. Using these 5-section signals causes the yellow trap hazard with those other sequences.
• Notice how the time periods when all lanes on an approach are moving at the same time are short. A way to lengthen these times of simultaneous flow would solve most of the problems.
• The eastbound and westbound legs are offset. Traffic must move over one lane within the intersection.
• The eastbound leg often backs up to over a mile to the west at rush hour.
• An eastbound right-turn green arrow cannot be displayed while the circular signals are red because the eastbound right-turn movement shares a lane with the eastbound straight-ahead movement. Being able to display such an arrow would alleviate the eastbound back-up.

Examining the Phase Sequence:

Phase Axy: Northbound and southbound left. This phase lasts only as long as both left turn lanes are full. May be skipped.			Cars trying to make these turns can be blocked by other cars waiting for the signal.
Phase Ax1: Northbound straight, left, and right. This phase services the two movements that lock together. But it does not last long enough to help the backed up vehicles.			Phase Ay2: Southbound straight, left, and right. This phase appears only if no traffic is waiting to turn left in the northbound leg.
Phase A12: Northbound and southbound thru and pedestrian. This phase (the coordinated phase) services major movements and associated crosswalks. Left and right turns are permissive.			Cars trying to make these movements can be blocked by other cars waiting to turn.
Phase Bxy: Eastbound and Westbound left. This phase lasts only as long as both left turn lanes are full. May be skipped.			Cars trying to make these turns can be blocked by other cars waiting for the signal.
Phase Bx1: Eastbound straight, left, and right. Services the high volume right turn and two movements that lock together. Does not last long enough to help backed up vehicles.			Phase By2: Westbound straight, left, and right. This phase can be skipped if not called. This phase appears only if no traffic is waiting to turn left in the northbound leg.
Phase B12: Eastbound and westbound thru and pedestrian. This non-actuated phase services east-west movements and associated crosswalks Left and right turns are permissive.			Cars trying to make these turns can be blocked by other cars waiting to turn.

The Effects on the Movements:

The Benefits:

1. A standard signal controller is used.
2. Standard quad-left signal sequence, detection, and display are used.

The Problems

1. Vehicles turning left and thru vehicles in the northbound lanes interfere with each other, because those movements usually occur at different times.
2. Vehicles turning right and thru vehicles in the eastbound lanes interfere with each other, because the stopped thru vehicles prevent right turning vehicles from turning right on red.
3. Vehicles changing lanes to go around a line of stopped vehicles encroach on the paths of other vehicles, causing crashes.
4. The backed-up traffic often extends back through other signalized intersections to the west.
5. The backed-up traffic keeps drivers on driveways along the south and west legs from making any turns other than right turns.
6. Cars waiting to make any eastbound movement or the northbound left turn may have to wait through multiple signal cycles.
7. The waiting in line and multiple stops will waste fuel and cause more pollution.

THE PROPOSED INTERSECTION REDESIGN:

DESCRIPTION

• The physical footprint of the intersection and its approaches is not to be changed.
• The left lane of the eastbound approach would carry both left-turn and straight-ahead traffic.
• The right lane of the eastbound approach would be reserved for right turns.
• The other approach lanes would have the same movements they have in the old configuration.
• All departure lanes would have the same movements they have in the old configuration.
• The left-turn signals for northbound and southbound traffic would be replaced with Flashing Yellow Arrow signal faces.
• The left-turn signal for westbound traffic would be replaced with a Flashing Yellow Arrow signal face. An additional 3-section circular signal face would be added to the right of the current 3-section face to provide the required two faces for the thru movement.
• The "doghouse" left-and-thru signal for eastbound traffic would be retained, because the eastbound left lane would be shared between left-turn and straight-ahead traffic. An additional right-and-thru "doghouse" face could be mounted to the right of the current 3-section circular face. An alternative is to change the right hand signal face to a thru-and-right doghouse face without adding a third face.
• The traffic signal sequence is changed to a split-lead-lag sequence for both streets. This sequence is shown in a fast animation to the right, and in a much slower animation showing the signal displays to the above right. Neither diagram shows any phase skips, though all four left turns can be skipped.
• An eastbound green right-turn arrow would be lit whenever the northbound green left-turn arrow is lit. A steady yellow arrow follows the green arrow. But the green arrow would not be lit during the eastbound circular green or circular yellow, to prevent second yellow trap. Right turning traffic has to pause to let oncoming left turns clear before the green arrow comes on.
• The jog in the eastbound straight-ahead movement is straightened out.
• The two lagging left turn phases are not called unless they are needed.
• If straight-ahead phases can be skipped, backup protection is needed to prevent phase 4 from following phase 5.

Examining the Phase Sequence:

Phase 1: Northbound straight, left, and right, with permissive southbound left. This phase can be held as long as needed to alleviate the backed up northbound left turns and westbound right turns. Since the northbound movements occur at the same time, the left turning cars do not block vehicles intending to make other movements.
Phase 2: Northbound and southbound thru and pedestrian, with permissive left and right. This phase services the major northbound and southbound movements and the associated pedestrian movements. Left and right turns are permissive, yielding to oncoming traffic and pedestrians. This is the coordinated phase.
Phase 3: Southbound straight, left, and right, with permissive northbound left. This phase services any left turning cars unable to turn during phase 2. It may be skipped if it is not needed. This is a lagging left turn phase, but yellow trap is prevented by the flashing yellow arrow display for northbound left turns.
Phase 4: Westbound straight, left, and right. This phase can be skipped if not called. It services waiting westbound left turns. Because eastbound left turns are in a shared lane, no permissive eastbound left turns are allowed. Because all westbound movements move at the same time during this phase, cars do not block each other.
Phase 5: Eastbound and westbound thru and pedestrian, with permissive left and right. This phase services the major eastbound and westbound movements and the associated pedestrian movements. Left and right turns are permissive, yielding to oncoming traffic and pedestrians. This is a non-actuated phase.
Phase 6: Eastbound straight, left, and right, with permissive westbound left. This phase services any left turning cars unable to turn during phase 5. It may be skipped if it is not needed. It services the high volume eastbound right turn. This is a lagging left turn phase, but yellow trap is prevented by the flashing yellow arrow display for northbound left turns.

The Effects on the Movements:

The Benefits:

1. Because most of the phases allow all of the movements on one approach to go at the same time, traffic should not become backed-up on the approaches.
2. The existing mast arms and many of the existing signal faces can be used.
3. Because cars stop fewer times and don't wait in long lines, less fuel is wasted and less pollution is generated.

The Problems

1. The signal wiring would have to be replaced, as more circuits are needed.
2. The signal controller might have to be replaced if it is too old.
3. Extra signal heads would need to be put up to maintain the required two signal faces for each thru movement. The old left-turn signals met the requirement, but the new left-turn signals do not display any thru movement indications.
4. The detection scheme will need to be changed to a lead-lag configuration.

RESULTS

The scores are simple attempts to calculate the traffic handling abilities of the movements.

Notice that the major movement score is much higher with the new design.

Since this has not actually been implemented, traffic counts would not make any sense.

HOW IT WOULD LOOK:

BEFORE

AFTER (simulation)

Links:

• All Yellow Trap Links are here:
  YELLOW TRAP INDEX PAGE
• Do you understand yellow trap?
• A Visual Example of Flashing Yellow Arrows
• Unusual Traffic Problems, Hazards, and Solutions
• Yellow Trap in Bloomington Indiana
• HOME