Consideration of Dual Left Turns at 82nd Street Intersections & at

50th Street Intersections

City of Lubbock Citizens Traffic Commission

By: Jere Hart

City Traffic Engineer

August 24, 2004/September 21, 2004

Dual Left Turn Locations• Dual left turn lanes (protected/permitted)

are currently installed at four intersections along 82nd Street:– 82nd and Slide– 82nd and Quaker– 82nd and Indiana– 82nd and University

• Three On 50th Street:– 50th (WB) & Slide– 50th (EB) & Q– Indiana (NB) & 50th

Advantages of Dual Left Turns

• Congestion Management

– Twice as much traffic is serviced compared to a single turn lane.

– The saved time helps relieve congestion for the entire intersection.

Disadvantages of Dual Left Turns

• The intersection is less safe

– Visibility becomes a problem for drivers using the new left turn lane.

Visibility Problems

Impact on Traffic Safety

• Accident reports from 2003 were compared for the Slide, Quaker, Indiana, and University intersections at 82nd Street.

• In 2003, only Slide had dual lefts installed – the others had single left turn lanes.

Traffic Safety Data

• 82nd and Slide 2003:22 of 29 applicable collisions were LT colliding

with oncoming ST traffic

• 82nd and Quaker 2001: 5 of 22 = 23%

• 82nd and University 2003:5 of 18 = 28%

• 82nd and Indiana 2003:5 of 15 = 33%

GraphRight Angle Collisions

28%33%

23%

76%

0

10

20

30

40

50

60

70

80

90

100

82nd @ University 82nd @ Indiana 82nd @ Quaker 82nd @ Slide

Location

Per

cent

age

of C

ollis

ions

Collision Diagram

Proposed Solution

• A possible solution to this problem is to prevent left turn traffic from making “permissive” turns (green ball).

• Traffic in the dual left turn lanes would be serviced only during the “protected” phase - when drivers see the green arrow.

Effects on Traffic Flow

• Intersection performance is measured in terms of a “level of service.”

• LOS values range from A through F, with A being the best.

Level of Service Values Simplified

• LOS A - Traffic flows freely; very little or no delay.

• LOS B - Good flow rate; slight delays.

• LOS C – Fair flow rate; light delays.

LOS D – Below average flow rate; moderate delays.

LOS E – Poor flow rate; heavy delays.

LOS F – Delays are severe. Most motorists consider this to be an unacceptable condition.

LOS Impact

Intersection Approach

82nd @ Slide EBLT

WBLT

NBLT

SBLT

82nd @ Quaker EBLT

WBLT

NBLT

SBLT

82nd @ Indiana EBLT

WBLT

NBLT

SBLT

82nd @ University EBLT

WBLT

NBLT

SBLT

Current Level of ServicePredicted New Level of Service after moving to "protected only" phasing

C

C

B

D

C

D

D

B

D

C

C

C

C

C

C

D

E

D

D

E

D

D

E

D

E

E

D

D

D

D

D

D

Extra Stops Required Per Year

IntersectionExtra vehicle stops required per year in “protected only” mode

82nd @ Slide 2.79 million

82nd @ Quaker 2.76 million

82nd @ Indiana 1.08 million

82nd @ University 1.81 million

Recommendation

The Traffic Engineering department recommends changing to “protected-only”

phasing for improved safety.

Will require additional left turn signal heads placed on far left side (2 heads per protected

direction) & New overhead sign:

Visibility Problems