Auxiliary Lane Components
Left-turn lanes are typically required when left-turn traffic approaches 50 vehicles per peak hour, or where needed for signal phasing and operation. Auxiliary lane components include deceleration length, approach taper, bay taper, and storage length.
On high-speed roadways, it may be desirable to design turn-lanes based on deceleration requirements rather than storage needs. This allows vehicles to gradually reduce their speed, thereby reducing the potential for rear-end collisions. Minimum deceleration lengths (including the taper length) for lanes with an accompanying stop condition are 235, 320, and 435 feet for design speeds of 30, 40, and 50 mph, respectively.
Use an approach taper to transition the through-travel lanes, and to obtain the necessary width to accommodate the left turn lane. The length of the approach taper is based on speed and width of the auxiliary lane to be added. This is calculated with the formula L=W*S2/60 for speeds of 40 mph or less, and L=W*S for speeds greater than 40 mph; where L is the approach taper length in feet, W is the width of the lane in feet, and S is the speed limit in miles per hour.
Storage length is based upon the maximum number of vehicles that will accumulate at any one time. This length should be long enough to accommodate all turning vehicles so that traffic doesn't back up into through-traffic lanes. Use a minimum storage length of 100 feet if the number of left turns per hour is 60 or less. Storage lengths may need to be increased based on deceleration requirements on high-speed highways.
Bay Taper Length
Bay taper length is the transition length from through-traffic lanes to adjacent auxiliary lanes. Bay taper lengths are dependent on vehicle speeds and width of auxiliary lane, although 100 feet is a common minimum. Typically, the bay taper length is 100 feet for speeds up to 30 mph and 180 feet for speeds of 35 mph and greater for a standard 12-foot width left-turn lane.
Driver Line of Sight
Driver visibility must be considered when selecting the location of turn left storage bays. Make sure appropriate left-turn sight distance, as discussed earlier, is available.