In the wake of his car crash on Thursday afternoon, in which the Duke of Edinburgh rather miraculously walked away unhurt, there has been much speculation over the man’s great age, and whether, at 97, he is now simply too old to operate so independently. Loss of autonomy becomes inevitable when you reach such a ripe vintage, but it is never easy to hand over – in both a metaphorical and literal sense – the keys you have held for so long in your grip.

My grandfather lived until he was 94. Neither he nor his wife, my grandmother (still alive at 99), looked, or acted, their age. The necessity to give up certain activities only ever came at them rudely, and they could never be told they were too old for anything; they had to learn it for themselves, the hard way.

One warm spring day in Milan, where they lived, I suggested that my wife and I might travel with them to Lake Como for lunch. It took several days of careful negotiation before my grandfather, then in his late 80s, would even consider the proposition – lunch in a restaurant was preposterous when the fridge contained food – but eventually he agreed. I offered to drive, but he scotched the idea. In England, we drove on the left, in Italy, the right. I couldn’t possibly, he insisted, cope with the reversal.

The road map suggested a journey of 45 minutes. After an hour and a half of barrelling down a newly built motorway in excess of the speed limit, it was safe to assume we had overshot it; there were now roadsigns ahead for Switzerland. If we didn’t turn off soon, we would be honeycombing the Alps. Repeatedly pleading with him to slow down, to turn right, had proved curiously ineffective. He just kept on going, his knuckles white. From the back seat, I could see a small rectangle of his face reflected in the rearview mirror. He looked unbearably tense.

It was only when my grandmother, sitting beside him, placed a hand on his arm as if to physically pull him into the next turn, that he acceded – but still he didn’t slow down, didn’t indicate. Behind us were horns, angry beeps. The car cried out in distress. We cried out, too. And still he drove on, still far too fast.

Eventually, he pulled up in a thoroughly nondescript town half an hour from home on the banks of a river, the front wheel mounting a kerb. The restaurants were closed, a single café open. It sold ice cream. We bought four. As he stalked up ahead, muttering to himself, his long-suffering wife explained that the roads had confused him, frightened him. She looked gaunt and distraught, but desperate that we empathise. “He’s old,” she said.

An hour later, drenched in sweat, we made it home. I wanted to shout at him, to remonstrate

Almost every day on Italian TV there are news reports about motorway pileups and reckless driving. By now, my grandfather seemed on his own collision course with his destiny – but I couldn’t understand it, couldn’t fathom why he would so carelessly risk our lives, and quite what had broken down in him so fundamentally.

On the way back, his silence remained thunderous, his anger expressed in the weight of his foot on the accelerator. He wanted this to be over as soon as possible, one way or the other. He wasn’t in his right mind, and wherever he was, we couldn’t reach him.

An hour later, drenched in sweat, we made it home. I wanted to shout at him, to remonstrate, but I knew that his own disappointment was greater than my anger could ever be. In the familiarity of their tiny flat, his universe righted itself, and at last he relaxed, and came back to us. I looked at him, this elderly and frail old man, as eminently fallible as the rest of us, but now more easily confused by the world outside; I held such affection for him then.

When we left for London a few days later, he didn’t offer to drive us the 15 minutes to the station, as he normally did. We’d walk. In fact, he rarely used the car after that, a valuable lesson belatedly learned. Prince Philip, with good reason, might be considering likewise.

The Smallest Things: On The Enduring Power of Family by Nick Duerden is published by Elliott & Thompson on 14 February