The M42 is a road of two parts. Its southern section forms part of the box of motorways around Birmingham, traversing the southern and eastern sides of the city and linking the M5 and M6; it then strikes off to the north-east, towards Nottingham and the East Midlands. The A42 is a direct continuation of the motorway route that carries traffic through to the M1.
Its purpose is to connect the East and West Midlands together, and to provide a route for traffic going longer distances between the north east and south west of England. In that role it replaces the A38, which was the previous route and which dumped all through traffic into the heart of Birmingham.
The route was completed in the late 1980s, with the A42 built last and at a slightly lower standard than the rest. It's often asked why the M42 didn't go all the way to Nottingham - as planned in the 1970s - and how it is that the number A42 was free to be allocated to its extension and not used elsewhere.
The original A42 was the main road from Oxford to Birmingham, which was swallowed up in the early 1930s by the extension of the A34 from Oxford to Birmingham and then Manchester. The A42 number became vacant and floated around for a long time without a home.
In the 1970s, a short section of motorway was built as a spur south from the M6 to the east of Birmingham, mainly to serve the NEC, and it was called M42. The choice of number may have been arbitrary, as "M42" was the next unallocated number beginning with a four, but the absence of an A42 may have influenced the choice as the new motorway wouldn't correspond to the route of any existing A-road. The motorway was progressively extended until it reached its current length.
Once the motorway reached the A444 at Measham, there were just fifteen miles between the end of the motorway and its destination, the M1 at Nottingham. But the appetite for motorway-building was waning as the 1980s wore on, and so the M42 ends here. A dual carriageway takes its place for the final fifteen miles, and they called it the A42.
Some engineers who worked on the A42 scheme describe instructions from senior civil servants that the overbridges should be built deliberately wide enough only for the roadway, with no space for hard shoulders to be added. The intention was to avoid bad publicity by ensuring the new road was categorically not a motorway.
Since then, an investigation into the route, submitted to the government in summer 2003, strongly recommended an upgrade of the A42 and M42 to provide a dual three-lane motorway all the way from Birmingham to the M1. but don't hold your breath for the roadworks to start: there's been no movement in the years that have passed since then.
In the early 2000s, the section to the east of Birmingham was the test bed for a new scheme to use technology to maximise the use of existing road space. It was called Active Traffic Management back then, but these days it's a common sight and it's known as Smart Motorways. It was also one of the first places you could see Driver Location Signs, which have since been introduced on motorways across England. You can be sure that if there's anything new on the motorway network, the M42 will have it first.