10.6 Left-Turn Lanes at Non-Signalized Intersections

  • Dedicated left-turn lanes at non-signalized intersections improve traffic flow and safety by removing left-turning vehicles from the through lane and providing space where they can decelerate await an acceptable gap before turning.
  • Adhere to AASHTO design recommendations for left-turn lanes. Even if establishing AASHTO-recommended lane lengths is not possible, left-turns lanes should be installed and kept as long as possible to minimize vehicle spillback into the through lane. Table 10.3 provides storage length calculations.
Table 10.3 Storage Length Calculations
US Customary Units Variable Definitions
Left Turn Capacity
    c = Left-turn capacity (vehicles/hour)
    V0 = Major-road volume conflicting with minor movement. Assumed to be one-half of the two-way major road’s volume.
    tᶜ = critical gap (sec)
    tᶠ = follow-up gap (sec)
Storage Length
    SL = storage length (ft)
    P(n > N) = probability of turn-lane overflow
    v = left-turn vehicle volume (vehicles/hour)
    c = left-turn capacity (vehicles/hour)
    VL = average length per vehicle (ft)
  • Avoid using negative offset left-turn lanes — they limit sight distance. On these configurations, a motorist turning left has their view of oncoming through traffic blocked by vehicles in the opposing turning lane (Figure 10.2). Visibility can be improved by using zero offset or positive offset.

Figure 10.2 Negative Offset Left-Turn Lane

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CONTACT:

Chris VanDyke

Research Scientist | Program Manager

chrisvandyke@uky.edu