I was excited to read Jam, intrigued by the premise: a ship of fools plot about a group of disparate unfortunates trapped on the M25, but have to say I was disappointed. The plot is paper thin. No reasons are ever given for the hold-up - presumably an accident up ahead, though it must be many miles up ahead as no one comes to tell the gridlocked multitudes what is happening; no helicopters hover overhead, no police or ambulances are seen racing to the scene on the empty other carriageway. To add to the misery, no one can get a signal on their mobile and the emergency roadside phones are out of order.
In short, you really do have to suspend more than a little disbelief at this predicament the characters find themselves in. In truth, it doesn't matter that much; this is a character led piece, a 'let's chuck these people who would never, in the normal course of things, get to meet, trap them together on a gridlocked motorway for hours and hours and see what happens' literary exercise. Your pleasure in reading Jam is mainly going to depend on how much you like/enjoy/sympathise with the characters, and in my case, I didn't much care for any of them except for Harold; I did like Harold very much, but he was the only one and he wasn't enough on his own to hold my interest.
It's not a terrible book, it's enjoyable enough; a quick and easy read, but there's precious little depth to it and there is no real conclusion; you never do find out what happens to these people once the traffic starts moving, the narrative does not tie up or finish, it merely ends. A good one for the airport or the beach, but not the weightier, meatier tome I was hoping for.