It was the highlight of the journey. Slowly, inexorably, beautifully, the counter clicked forward that additional digit from 61,999 miles. My eyes fastened on the three cream zeroes rising into view, and I reflected on the engineering triumph that is a car. The majesty of the thing, the sheer elegance. My Ford Fiesta.

My editor's idea was simple: retrace the route of the marathon in a car, see whether the traffic is as bad as people say it is. Don't speed, don't break the lights, but don't hang about.

He didn't tell me I'd have to spend so much time cooped up in a tin box, peering out at the world through smudged windscreens, biding my time at nigh on 100 sets of traffic lights. Even the tape player was on the blink.

The omens for a fast ride were good. There were no farming protests last Thursday, no impromptu sit-downs by building workers. It was mid-term, so most of the schools traffic was off the road. The weather was the same as on marathon Monday, bright and cold and blustery, but dry.

At kilometre zero, in O'Connell Street, the balloons that marked the start of the marathon had been replaced by the more familiar sight of buses, trucks and cars. Not too many, though, the rush hour was over.

Five minutes into my journey, however, I was still in O'Connell Street. First, there was a truck on the junction box with Abbey Street, which blocked all progress for a few minutes. On the quays heavy trucks rumbled in both directions, empty ones to the port, full ones heading for west Dublin and the country.

Light though the traffic was, the car was proving no faster than running. The Grand Canal arrived after 10 minutes, the RDS after 15; both roughly the same times it took to travel by Shanks's mare. Merrion Road passed smoothly, but the right turn on to Nutley Lane was slow, and trucks and cars were blocking the exit from the Merrion Shopping Centre.

Pat Kenny was discussing ideas for energy-saving on the radio as I breezed down the Stillorgan Road, but again a right filter to Foster Avenue caused lengthy delay. This was the pattern for the journey, steady progress in light traffic, punctuated by considerable delays linked to right turns.

But there were other causes for the hold-ups. In many places impromptu roadworks blocked off lanes, causing long tailbacks. More often vans and cars parked illegally on double yellow lines or bus lanes.

Once someone started the trend, he or she was quickly followed by copycat malefactors, and the lane was lost to traffic. On some streets, for example Baggot Street, delivery trucks would then park parallel to these cars, thereby blocking off a second lane.

Often, drivers left their warning lights blinking, as though this were some sort of explanation or expiation or even a device to ward off traffic wardens or gardai.

It took a full hour to wheel back towards the city centre again and pass the marathon's 10-mile mark in The Coombe. At this stage on Monday I was running comfortably, full of the joys of sunshine and fresh air. On Thursday I was getting edgy, tired of the interminable traffic lights, bored with the radio. How do people do this every day?

But I was making progress. The halfway point arrived near Kimmage Cross after one hour and 19 minutes; it took me 1:41 to run this. Through a succession of housing estates and suburban roads, the car continued to assert its ascendancy, although its driver was beginning to grow cramped. A bit like race day, funny enough.

Most of my fellow drivers were alone in their cars. Some seemed to be singing, others were lost in mobile phone calls, but most seemed to be in a trance. I knew how they felt.

Still, the finishing line was in sight. Chapelizod Road flashed by and a record time for a Ford Fiesta beckoned.

Unfortunately, I hadn't reckoned with a monster snarl-up on the quays. Truck after truck backed up, all heading for the ferry. The last three miles took more than half an hour, no quicker than a mild jog.

Finally, after two hours and 31 minutes and almost 100 sets of traffic lights, O'Connell Street arrived once more. That's a bit over 10 miles an hour, and 56 minutes faster than this average hack runner managed for the distance. Or 11 minutes slower than the winner, Kenya's Joshua Kipkemboi. Fiesta, Ford would have taken sixth place on the day with that time.

The majesty of the thing, the elegance. My Ford Fiesta. For sale.