The B409 is a short B-road in west London whose route seems to have been planned to take in as many different London boroughs as possible given its short length (three in about a mile).
It starts at a mini-roundabout on the B490 on the northern side of Acton Green Common, where that road turns left from Acton Lane into Beaconsfield Road. It heads east along South Parade, skirting the edge of the Common. At the end of the road it meets the B491 and there there is a short multiplex south along that road between two mini-roundabouts.
The B409 regains its number and continues east along Bath Road and Stamford Brook Road to end on a roundabout on the A402 Goldhawk Road.
Originally the section of the B409 along Acton Green Common was unclassified. At the western end of Bath Road it turned south past Turnham Green tube station to end on the A4 (now A315) Chiswick High Road. This section was renumbered B491 when the B409 was rerouted west.