Synthesis of Safety for Traffic Operations - Transports Canada

Intersection Control
Shebeeb (1995)
Shebeeb (1995) studied the safety of various left-turn phasing using crash rate as the
primary variable. The study included 54 intersections from Texas and Louisiana, and was
conducted on approaches, as phasing varied among approaches to the same intersection.
All of the subject intersections had exclusive left-turn lanes. Three consecutive years of
crash data was used in the analysis. The left-turn phasing studied included 3 :
• Permissive Only
• Lead Protected – Permissive
• Lag Protected – Permissive
• Lead Protected Only
• Lag Protected Only
Crash rate was determined through Equation 3.17.
Crash rate =
A * 10 6
V OP V LT
[3.17]
where:
A = total number of left-turn crashes
V OP = Volume of opposing vehicles (straight and right turn) (vph)
V LT = Volume of left-turning vehicles (vph)
The results of the analysis are shown in Table 3.49.
TABEL 3.49: Safety Records of Left-turn Phases
Phasing
Number of Mean Crash Standard
Approaches Rate Deviation
Permissive Only 38 49.2 94.3
Lead Protected –
Permissive
40 35.6 70.9
Lag Protected –
Permissive
23 61.2 85.9
Lead Protected Only 45 16.7 26.8
Lag Protected Only 13 21.7 30.6
Statistical tests of significance were applied to the crash rates, assuming normal
distribution. The following conclusions were reached:
3 Two additional left-turn phases referred to as Lead Dallas and Lag Dallas phases were also studied. They
are not presented herein.
Page 54