The M45 is a spur from the M1 in Northamptonshire, westward towards Coventry. It's a short motorway with a surprising amount of history.
Today it just provides access to part of Coventry from one direction on the M1, and consequently it is, perhaps, not the most important motorway in Britain. But its historical significance is much greater: it was the original northern spur from the M1, and for a number of years it was part of the main road from London and the south-east to the West Midlands and the north.
It was built to connect the brand-new M1 to the A45 towards Birmingham, and performed a vital role in distributing traffic coming off the new motorway across the existing road network of the Midlands. But its heyday lasted only about twelve years. In 1971, the parallel section of M6 opened, connecting directly to the M1, and since then it's been a backwater. Once part of the signposted route from London to Birmingham and Coventry, today the signs on the M1 just point to "Coventry (S)". Its appearance is a little delapidated these days.
In its original form, the M45 was very basic, merging straight into the M1 at the eastern end, and taking a non-stop route to a roundabout on the A45 at the other. This left traffic on the A45 to pass through the village of Dunchurch, even though the motorway ran right past in parallel. In the 1980s, the village was bypassed by building a new junction with the A45 east of the village, allowing through traffic to use the motorway. Oddly, considering the terminal roundabout is grandly numbered junction 1, this junction has no number. Perhaps "0A" was not considered a suitable number for the road signs.