The M5 is a unique motorway. Unlike its other single-digit friends 1-6 it doesn't radiate from London and its purpose is not just to connect major industrial areas. How busy it is depends on what time of year it is.
The reason it exists (besides the everyday, humdrum job of connecting the places along its length) is to carry holiday traffic headed for the south west - Devon and Cornwall. This is the road to the seaside.
It picks up traffic from the north west first (M6), then the east Midlands and north east (M42), then south east (M4), and carries it all south west. Then, at Exeter, it gives up, distributing its traffic onto the A30 to Cornwall, the A38 to south Devon and the A380 to east Devon.
That's not to say that the M5 isn't a working motorway; far from it. The northern part of its route links Birmingham to Bristol, and along with the lengthy M50 spur, it links the industry and ports of Avonmouth and South Wales to the Midlands and the North. It's a busy road.
Initially the section between present-day M42 and M50 was just two lanes each way. When the rest was built on either side - all with three lanes - this part became a huge bottleneck. The widening works took years, moving on from one section to the next and making the already-chronic congestion noticeably worse. The resulting road is excellent, but it could have been avoided if the M5, like most other motorways at the time, had been built with three lanes each way from the outset - saving massive disruption and an awful lot of money.
Its problems extend further south, too; not long after the widening was done, the Avonmouth Bridge near Bristol became the trouble spot and for almost a decade contractors worked to widen it to four lanes each way. The section around Bristol remains the most problematic, with new climbing lanes recently added to alleviate traffic jams on its steep hills. With no other reasonable way past the city, it is likely to remain a headache for holiday traffic for some time to come.