Driven crazy: David Whitley

There's a horrible clunking sound as we go down the hill: a symphony of car bottom scraping against rocks, interspersed with creaking howls of protest from the suspension. We wince, hoping no damage has been done. And, if there has, it'd better be hidden well enough for the rental car company not to notice.

In the baking heat of Monument Valley, straddling the Utah/Arizona border, we've ticked off the second ingredient of any proper US road trip: an incident that will leave you fretting for the rest of the journey (and a good couple of months after you get back). The first ingredient had come about an hour earlier, as we descended into the cinematic desert valley of towering rock formations. That, naturally, is an animated debate over whether you're allowed to take the car down a dirt track. I'm sure someone checks this sort of thing on their rental agreement, but that someone is most certainly not me.

These aren't the only staples of a successful road trip, of course. One lunchtime, there will be a breaking-point tantrum where you can't bear to eat another taco, burger or Subway sandwich. There will also be a night where 'a little drink in the motel room' turns into voracious consumption of every alcoholic substance you've stashed in the car boot. By law, this has to take place the morning before an early start and a really long drive. There also has to be at least one day where you seriously miscalculate the driving time, and limp exhausted into bed a good three or four hours later than expected.

Being in the US adds a few extra foibles. There will be the moment you exit a car park and realise you're on the wrong side of the road. There will be the point you get off the freeway at the wrong junction as no one will let you out of the 'must exit' right-hand lane. And there will be two days of flitting between country and Christian rock radio stations before settling on a lunatic right-wing talk radio in a state of permanent outrage.

Yet despite all this, there comes a point where the inherent madness of driving four or five hours a day and changing accommodation every night starts to feel inherently rational. There's no sense of dread about having to get back in the car again, and it stops becoming about the key attractions you're trying to string a route between. You start getting increasingly eager to set the alarm an hour earlier and wangle in diversionary jaunts down back roads. You develop an absurd 'in my day it were nowt but fields'-style stoic pleasure from putting in increasingly long stints behind the wheel. The definition of 'too far out of the way' begins to gain extraordinary elasticity.

After two-and-a-half weeks of thundering through the southwestern deserts, there's a howling, foot-stamping reluctance to return to normality. But first, there's the other ingredient to deal with: a squabble about whether to wash the car before it's returned. After 18 days without a single drop of rain, during which there's been the odd dalliance with a dirt road, the windscreen is now the world's most prominent insect graveyard; and the car, in general, is a filthy mess.

Frankly, it should have been washed days ago, as we can barely see out of the front for squished dragonflies and bird muck. But no one ever knows quite how far you can push the definition of 'reasonable condition' when returning a car.
As ever, the decision to not wash it is based on cynical logic. Dirt is very, very good at disguising any scratches and dings we may have picked up. By the time the rental firm has washed it, it'll be far too late to pull us up over any minor redecorations. The exterior can be left as it is. The interior is a different matter. The best measure of how great a road trip has been is the state of the back seat. The more shame-inducingly appalling, the better. The empty water bottles, coffee cups, crisp packets and Twix wrappers thrown over the shoulder would give Wombles a heart attack. But they also serve as easily the most scientific measurement. As well as creating the final key ingredient: trying to fit it all into a petrol station bin without anyone witnessing the industrial-grade dumping.

Follow @mrdavidwhitley

Published in the March 2014 issue of National Geographic Traveller (UK)