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Section 7: Auxiliary Lanes

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Overview

This section describes the basic use and functional criteria associated with auxiliary lanes. Auxiliary lanes consist of left-turn and right-turn movements, deceleration, acceleration, and their associated transitions and storage requirements. Left-turn movements may pose challenges at driveways and street intersections. They may increase conflicts, delays, and crashes and often complicate traffic signal timing. These problems are especially acute at major highway intersections where heavy left-turn movements take place, but also occur where left-turn movements enter or leave driveways serving adjacent land development. As with left-turn movements, right-turn movements pose problems at both driveways and street intersections. Right-turn movements increase conflicts, delays, and crashes, particularly where a speed differential of 10 mph or more exists between the speed of through traffic and the vehicles that are turning right.

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Functional Criteria

Table 2-3 presents thresholds for auxiliary lanes. These thresholds represent examples of where left turn and right turn lanes should be considered. Refer to the TxDOT Roadway Design Manual Chapter 3, for proper acceleration and deceleration lengths.

Anchor: #i1002998Table 2-3: Auxiliary Lane Thresholds

Median Type

Left Turn to or from Property

Right Turn to or from Property (5)

-

Acceleration

Deceleration

Acceleration

Deceleration

Non-Traversable

(Raised median)

(2)

All

Right turn egress > 200vph (4)

  • > 45mph where right turn volume is > 50vph(3)
  • ≤45 where right turn volume is > 60vph(3)

Traversable

(Undivided Road)

(2)

(1)

Same as above

Same as above

(1) Refer to Table 3-11, TxDOT Roadway Design Manual, for alternative left-turn-bay operational considerations.

(2) A left-turn acceleration lane may be required if it would provide a benefit to the safety and operation of the roadway. A left-turn acceleration lane is generally not required where the posted speed is 40 mph or less, or where the acceleration lane would interfere with the left-turn ingress movements to any other access connection.

(3) Additional right-turn considerations:

  • Conditions for providing an exclusive right-turn lane when the right-turn traffic volume projections are less than indicated in Table 2-3:
    • High crash experience
    • Heavier than normal peak flow movements on the main roadway
    • Large volume of truck traffic
    • Highways where sight distance is limited
  • Conditions for NOT requiring a right-turn lane where right-turn volumes are more than indicated in Table 2-3:
    • Dense or built-out corridor where space is limited
    • Where queues of stopped vehicles would block the access to the right turn lane
    • Where sufficient length of property width is not available for the appropriate design

    • (4) The acceleration lane should not interfere with any downstream access connection.

  • The distance from the end of the acceleration lane taper to the next unsignalized downstream access connection should be equal to or greater than the distances found in Table 2-2.
  • Additionally, if the next access connection is signalized, the distance from the end of the acceleration lane taper to the back of the 90th percentile queue should be greater than or equal to the distances found in Table 2-2.

  • (5) Continuous right-turn lanes can provide mobility benefits both for through movements and for the turning vehicles.1 Access connections within a continuous right turn lane should meet the spacing requirements found in Table 2-2. However, when combined with crossing left in movements, a continuous right-turn lane can introduce additional operational conflicts.




1. Florida Department of Transportation (FDOT), Florida’s Driveway Handbook, 2002.

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