the + highway number

[Damien Hall]

British highways are only ever referred to with a definite article;  so we have

the M5 ('M' is for 'Motorway', 'Interstate' equivalent)
the A1
the B416

(I suppose A-roads would be the equivalent of State Routes or less-busy sections
of Interstates - they don't to my knowledge ever have any property directly
adjoining them except where they pass through towns, in which case their
speed-limit is reduced.  B-road is the classification below A-road:  a fairly
major road which usually has sidewalks / unfettered access from whatever's
alongside it if not buildings, and a correspondingly lower speed-limit.)

The tendency to name roads using definite articles even extends to putting them
in front of proper nouns which name roads if the roads are fairly large or
important, even if they are also in residential / built-up areas.  So the road
my parents live on is 'Argyle Road' on maps and in addresses, but 'the Argyle
Road' when you're giving directions for people to drive along it.

Damien Hall
University of Pennsylvania