FIXING A BAD INTERSECTION

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

THE INTERSECTION AS IT IS:

Intersection of Walnut and Grimes DESCRIPTION

THE PROBLEMS

ANALYSIS

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.

northbound thru and left
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.

northbound thru and left
southbound thru and left
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.

northbound and southbound thru
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.

northbound thru and left
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.

eastbound thru and left
westbound thru and left
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.

eastbound and westbound thru
Cars trying to make these turns can be blocked by other cars waiting to turn.

The Effects on the Movements:

TRAFFIC MOVEMENT SCORES PER MOVEMENT
TRAFFIC MVT# LANES Φ STOPΦ RTOR Φ PERMΦ PRIV SCORE
Northbound Left1 Excl ( 0) ... Ay2, B( 0) ... -( 3) ... A12( 8) ... Axy, Ax111
Northbound Straight2 Excl ( 0) ... Axy, Ay2, B( 0) ... -( 0) ... -(11)... Ax1, A1211
Northbound Right1 Shar ( 0) ... -( 3) ... Axy, Ay2, B( 4) ... Ax1, A12( 0) ... -7
East Crosswalk1 Xing ( 0) ... Axy, Ay2, B( 0) ... -( 0) ... -(10)... Ax1, A1210
Southbound Left1 Excl ( 0) ... Ax1, B( 0) ... -( 3) ... A12( 6) ... Axy, Ay29
Southbound Straight2 Excl ( 0) ... Axy, Ax1, B( 0) ... -( 0) ... -(11)... Ay2, A1211
Southbound Right1 Shar ( 0) ... -( 3) ... Axy, Ax1, B( 3) ... Ay2, A12( 0) ... -6
West Crosswalk1 Xing ( 0) ... Axy, Ax1, b( 0) ... -( 0) ... -( 8) ... Ay2, A128
Eastbound Left1 Shar ( 0) ... By2, A( 0) ... -( 3) ... B12( 6) ... Bxy, Bx19
Eastbound Straight1 Shar ( 0) ... Bxy, By2, A( 0) ... -( 0) ... -( 8) ... Bx1, B128
Eastbound Right1 Excl ( 0) ... -( 3) ... Bxy, By2, A( 3) ... Bx1, B12( 0) ... -6
South Crosswalk1 Xing ( 0) ... Bxy, By2, A( 0) ... -( 0) ... -( 8) ... Bx1, B128
Westbound Left1 Excl ( 0) ... Bx1, A( 0) ... -( 3) ... B12( 6) ... Bxy, By29
Westbound Straight1 Shar ( 0) ... Bxy, Bx1, A( 0) ... -( 0) ... -( 8) ... By2, B128
Westbound Right1 Shar ( 0) ... -( 3) ... Bxy, Bx1, A( 3) ... By2, B12( 0) ... -6
North Crosswalk1 Xing ( 0) ... Bxy, Bx1, A( 0) ... -( 0) ... -( 8) ... By2, B128
 
Major Movements Score(cyan rows)   39
Minor Movements Score(white rows)   62
Vehicle Movements Score(cyan and white rows)   101
Pedestrians Score(orange rows)   34
Total Score(cyan, white, and orange rows)   135
 
SCORING VALUES PER PHASE
LANESΦ TIMESTOP RTORPERMPRIV
2 ExclusiveLong0136
1 ExclusiveLong0135
1 SharedLong0.525
2 ExclusiveShort0125
1 ExclusiveShort0123
1 SharedShort0.513

Phases Ax1, A12, and B12 are long.

Phases Axy and Ay2 are short.

Phases Bxy, Bx1, and By2 are short.

Partial scores at left are in parentheses.

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:

New Intersection of Walnut and Grimes DESCRIPTION

ANALYSIS

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.

northbound thru and left
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.

northbound and southbound thru
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.

southbound thru and left
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.

westbound thru and left
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.

eastbound and westbound thru
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.

eastbound thru and left

The Effects on the Movements:

TRAFFIC MOVEMENT SCORES PER MOVEMENT
TRAFFIC MVT# LANES ø STOPø RTOR ø PERMø PRIV SCORE
Northbound Left1 Excl ( 0) ... 4, 5, 6( 0) ... -( 5) ... 2, 3( 5) ... 110
Northbound Straight2 Excl ( 0) ... 3, 4, 5, 6   ( 0) ... -( 0) ... -(12)... 1, 212
Northbound Right1 Shar ( 0) ... -( 2) ... 3, 4, 5, 6   ( 4) ... 1, 2( 0) ... -6
East Crosswalk1 Xing ( 0) ... 3, 4, 5, 6( 0) ... -( 0) ... -(10)... 1, 2   10
Southbound Left1 Excl ( 0) ... 4, 5, 6( 0) ... -( 6) ... 1, 2   ( 3) ... 39
Southbound Straight2 Excl ( 0) ... 1, 4, 5, 6( 0) ... -( 0) ... -(11)... 2, 311
Southbound Right1 Shar ( 0) ... -( 2) ... 1, 4, 5, 6( 3) ... 2, 3( 0) ... -5
West Crosswalk1 Xing ( 0) ... 1, 4, 5, 6( 0) ... -( 0) ... -( 8) ... 2, 38
Eastbound Left1 Shar ( 0) ... 1, 2, 3, 4( 0) ... -( 2) ... 5( 3) ... 65
Eastbound Straight1 Shar ( 0) ... 1, 2, 3, 4( 0) ... -( 0) ... -( 8) ... 5, 68
Eastbound Right1 Excl ( 0) ... -( 3) ... 2, 3, 4( 5) ... 5, 6( 5) ... 113
South Crosswalk1 Xing ( 0) ... 1, 2, 3, 4( 0) ... -( 0) ... -( 8) ... 5, 68
Westbound Left1 Excl ( 0) ... 1, 2, 3( 0) ... -( 5) ... 5, 6( 3) ... 48
Westbound Straight1 Shar ( 0) ... 1, 2, 3, 6( 0) ... -( 0) ... -( 8) ... 4, 58
Westbound Right1 Shar ( 0) ... -( 2) ... 1, 2, 3, 6( 3) ... 4, 5( 0) ... -5
North Crosswalk1 Xing ( 0) ... 1, 2, 3, 6( 0) ... -( 0) ... -( 8) ... 4, 58
 
Major Movements Score(cyan rows)   46
Minor Movements Score(white rows)   54
Vehicle Movements Score(cyan and white rows)   100
Pedestrians Score(orange rows)   34
Total Score(cyan, white, and orange rows)   134
 
SCORING VALUES PER PHASE
LANESø TIMESTOP RTORPERMPRIV
2 ExclusiveLong0136
1 ExclusiveLong0135
1 SharedLong0.525
2 ExclusiveShort0125
1 ExclusiveShort0123
1 SharedShort0.513

Phases 1, 2, and 5 are long.

Phases 3, 4, and 6 are short.

Partial scores at left are in parentheses.

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.

Traffic MovementOld DesignNew Design
Major Movements Score3946
Minor Movements Score6254
Vehicle Movements Score101100
Pedestrians Score3434
Total Score135134

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:

Old Walnut and Grimes

BEFORE

New Walnut and Grimes

AFTER
(simulation)

Links: