Turn lane length = deceleration + storage
(+ taper per standard): decel from FDM tables by design speed #status/verify FDM section; storage ≥ 95th-percentile queue (Synchro/HCM/SimTraffic per agency preference) and ≥ agency minimum.
Warrant to ADD a turn lane
Volume-based thresholds (FDM/NCHRP-derived charts; agency guidelines) — check before designing length.
Queue checks that bite
- 95th queue vs storage bay (spillback into through)
- vs upstream driveway/intersection spacing
- internal site queues at gates/drive-thru vs throat length (throat length is the #1 site-plan comment — measure from ROW to first internal conflict)
Worked habit
One-page calc per movement: volumes → queue source → storage provided vs required → verdict.
Practice question
Dual lefts, 95th queue 310 ft, bays 250 ft, 200 ft to upstream driveway — enumerate the failures and two fixes.
QA/QC reminder
Queue SOURCE named (Synchro 95th? HCM? SimTraffic max?) — mixing sources across a table is an audit finding.
See also: FDOT Design Manual (FDM) · Workflow - TIA from Request to Submittal · Synchro QAQC Checklist