Turn Lane Length Calculator

A turn lane isn't just a wider stretch of pavement -- it is three distinct engineered segments stitched together: a deceleration length where a driver slows from through-traffic speed before entering the lane, a storage length long enough to hold the design queue of turning vehicles without spilling into the through lane, and a taper length that guides drivers smoothly into the new lane. This calculator sizes all three per AASHTO Green Book guidance. Deceleration length follows basic kinematics -- stopping distance grows with the square of approach speed, so raising speed from 25 to 61 mph (a 2.4x increase) drives roughly a 6x jump in deceleration length on its own. Storage length depends on whether the intersection is signalized: for a signal, it estimates the average queue that accumulates during the red phase of the turning movement, scaled up to a 95th-percentile design queue; for an unsignalized intersection (enter a Signal Cycle Length of 0) it instead estimates queuing from the turning volume and an assumed gap-acceptance delay.

Taper length comes from a standard taper-rate table that steepens in stages as approach speed climbs past 30, 40, and 50 mph. The Total Length (with taper) output sums all three segments and is the number to carry into a striping or design drawing. This is a planning-level tool, not a substitute for a full traffic engineering study using local counts and an agency-approved design manual.