In the tradition of films and books where strangers are thrown together in a confined space, be it plane, ship or lift, Jake Wallis Simons corrals an assortment of humanity in a traffic jam and takes the reader into the stationary cars to introduce their increasingly frustrated occupants.

The time is a Sunday evening, the place the M25 (or London Orbital), famous as probably the most congested and loathed of English roads. Not everyone hates it though, as we discover in this digressive, personable novel- one of those caught up in the jam confessing that he sometimes drives around it several times at a go, in his camper van. In the opinion of this Scottish history professor, it is a magical place, and so he makes it sound, with snippets of history of the kings and Romans who once lived here. Today's snaking circuit, he says, is "like a continual stream of transience, permanent and primordial, like a river. Only made by man, that's the thing."

But for everyone else on this benighted evening, the M25 is little short of hell. Not sure what has brought traffic to a standstill - with police cars zooming past on the hard shoulder, helicopters overhead and no mobile phone reception - people can do nothing but sit and wait until the hold-up ends.

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There are Max and Ursula, whose marriage is at breaking point, with their daughter in the back seat with a friend, both screaming to be allowed to watch telly to pass the time. "Carly opened her eyes. 'I want TV,' she mumbled. 'Jesus H Christ,' said Max. 'I want the iPhone,' said Bonnie. 'I want TV. Fuck head.' 'Right!' shouted Max. 'Both of you shut up! Shut up! I want total silence! Total silence, OK? Silence.' At this, both of the girls began to wail."

Then there's the Waitrose van driver, Jim, a sitting duck as tempers fray and stomachs rumble, who comes under pressure to open his van and share its contents.

Since no British road would be complete without its white van, Simons includes one with a more sinister purpose than most. It contains an undercover cop who is infiltrating an English supremacist group, one of whom - Rhys, innit? - is a human rottweiler, straining at the leash for a fight. His ideal prey are in another car: three middle-class Asian Londoners who unwisely emerge from their vehicle to play football on the silent motorway sometime after midnight, when fog and testosterone and a lethal brand of prejudice combine with predictable results.

Having set his stage, Simons builds up the story as tensions rise, arguments explode or, among the less volatile, the endless hours are used to contemplate what they're doing wrong in life, and how they'd like things to change. Unlikely friendships are struck, new resolves are made, some that could prove life-changing.

A sentimentalist at heart, Simons likes happy endings. Oddly, though, the least eventful portrait is also the most amusing and affecting, that of the professor and a young academic whose specialty is insects, and the nutritional value thereof. As the pair take tea in the back of the camper van, one senses the writer heaving a sigh of relief at a moment's respite from the high drama out on the road.

All of this is enjoyable, but there is an innate and intractable problem with Simons's setting which he proves unable to surmount. As a location, the jam is static and dull, and there is only so much flashback he can introduce to change the scene and fill in his characters' stories. Even more troublesome is the slowness with which he unfurls the narrative, and his reliance on tracking between characters, some inevitably more fascinating than others. A near-fatal longueur afflicts the book mid-way, episodes feeling repetitive, as are conversations, the plot inching forward almost as if caught in a jam of its own.

A ruthless editor could have stripped this book by a third, lost little by way of content and gained much in momentum. But then it would be a different novel. Simons's charm lies in the loving attention he devotes to his characters. This may explain the rather abrupt ending, as if he cannot bring himself to part from them, and thus gets it done quickly, with barely a backward glance.