|Distance:||2.7 miles (4.3 km)|
|Met (1956):||A4, B379, A4|
|Route outline (key)|
The B3378 came into being in the late 1920s following construction of the A4 Colnbrook bypass to take over the pre-bypass route. Although the road is still in existence it was declassified in the 1970s.
The road turned off the A4 on the southern edge of Harmondsworth and headed west through Longford along Bath Road. This road now lies very close to the perimeter of Heathrow Airport, perhaps a reason why the road was downgraded. Today we pass some airport hotels and then a bus gate for westbound traffic. After running through the centre of Longford the road crossed the River Colne. After World War II it then met the B379 (later upgraded to A3044) which was rerouted to meet this road when Heathrow Airport was built on its original route.
The road continued west over a level crossing by Colnbrook station and shortly reached Colnbrook itself. It ran along the High Street before leaving town along Horton Road and ending back on the A4. There's a roundabout here now.