Minimum speed-change lane lengths per AASHTO 2001 Green Book (Tables 10-4 and 10-6), with grade adjustment factors (Table 10-5). Based on Average Running Speed differentials between highway and entry / exit curve.
Based on AASHTO 2001 formula using Average Running Speeds (ARS):
L = (ARSH² − ARSE²) / 41.47 [adec = 1.6 m/s²], rounded to nearest 5 m.
Verified: VH=100, Vexit=40 → ARS 85 vs 35 → L = 145 m ✓
Highway Design Speed shown across top; Exit Curve Design Speed down left. highlighted = current selection.
| Design Speed (kph) | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 | 130 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg Running Speed (kph) | 20 | 28 | 35 | 47 | 54 | 62 | 70 | 77 | 85 | 91 | 97 | 102 |
ARS values from AASHTO 2001 Exhibit 3-1. The speed-change lane length is based on the difference between the Average Running Speeds of the highway and the entry/exit curve — not the posted design speeds.
Deceleration: Negative grade (downgrade) opposes braking → factor > 1 (longer lane required).
Positive grade (upgrade) — AASHTO takes no credit; factor = 1.00 (no reduction).
Acceleration: Positive grade (upgrade) opposes acceleration → factor > 1 (longer lane required).
Negative grade (downgrade) — AASHTO takes no credit; factor = 1.00 (no reduction).
A speed-change lane is an auxiliary lane provided at interchanges and driveways to allow vehicles to accelerate (entering) or decelerate (exiting) without disrupting mainline traffic flow. AASHTO 2001 Green Book §10.9.6 requires them wherever the speed differential between the highway and the connecting road exceeds approximately 15–20 kph.
The formula uses Average Running Speeds (ARS), not posted design speeds: L = (ARSH² − ARSE²) / 41.47, where the divisor comes from twice the deceleration rate (a = 1.6 m/s²) times 25.92 (unit conversion). The result is rounded to the nearest 5 m and checked against the 15 m minimum. ARS values are taken from AASHTO Exhibit 3-1.
AASHTO 2001 is conservative on favorable grades. For deceleration, an upgrade does assist braking, but AASHTO does not permit a reduction below the level-grade length — the grade factor is held at 1.00. Only unfavorable grades (downgrades for deceleration, upgrades for acceleration) require an increase in length per Table 10-5.
Design speed is the maximum safe speed for the geometric features of a road. Average Running Speed is the mean speed of traffic under free-flow conditions, always equal to or less than design speed. AASHTO uses ARS (from Exhibit 3-1) to compute speed-change lane lengths because it better represents actual vehicle behaviour during the merge or diverge manoeuvre.
This tool uses metric units (kph and metres) consistent with the metric edition of AASHTO 2001. For US customary units (mph and feet), the formulas and table values differ. Designers working in imperial units should refer to AASHTO Tables 10-4, 10-5 and 10-6 in the US customary edition and use the corresponding divisors (adec = 0.977 ft/s², aacc = 0.488 ft/s²).