The promise was grand and manifold: a real-world locale, a vehicle list filled with exotica, thousands of kilometres of open road and a thriving population of actual humans to race against. For the most part, Test Drive Unlimited delivered on its promises, but Eden Studio's greatest game set a milestone that few racing titles even dare to attempt: allowing virtual driving to be an open-ended, self-determined and leisurely pursuit.

It seems deceptively simple: you can just go for a drive. Whether that's a long voyage or short blast was up to you. The game's 1500 square kilometres of Hawaiian landmass and its vast and intricate system of roads meant the routes for driving really were unlimited, given the average human lifespan. Certainly, there had never been a game where you could set a waypoint on a huge map and spend an hour getting there in a real-world performance car. There had also never been a game where you could do this with pals in tow, merrily chatting along the way.

When I got my copy, the first thing I did was grind away at race events until I could afford my first 'proper' sports car, the lovely Lotus Esprit V8. From my lowly starter house, I set off for the lighthouse in Oahu's northwest corner. It took me three quarters of an hour to arrive there (with a fair amount of offroad excursions and wince-inducing prangs), but pulling up in the lighthouse's car park and swinging the in-car camera around to look out to sea felt like I had achieved something - and that something felt very, very new and exciting.

The glorious Lotus Esprit V8. A cheap thrill that drives early progress.

It was the joy of driving for the sake of driving, to feel the thrill of pushing the simulated machinery to the car's limits and my own, and without gamey intrusions to muddy the experience. Even if the smoothness of the journey was subject to the horrors of a handling model that was "unique" (to be diplomatic about it), it wasn't so much about the quality of the simulation as it was the freedom to be driving somewhere, anywhere, and going fast. I could just see where the road takes me by arbitrarily deciding where to go at each junction, until I ended up in some far-flung endpoint with the sense of satisfaction that I'd really thrashed the car silly and racked up a decent amount of miles on the odometer.

Nothing has matched Test Drive Unlimited for this - not even Forza Horizon with its vastly superior handling, as it has neither the freedom nor the scale of environment to offer the same proposition. You can't even get your own house.

For the free-form driver, Eden's Oahu still offered challenges, albeit more leisurely ones. Having to drive to car dealers, shops and real estate agents to access them sets an initial task list, and the process revealed Oahu's varied terrain, spurring further exploration. Driving in unexplored areas contributed to the long-game achievement of uncovering all the island's roads - an epic task by any standard, but one that was always ticking away unobtrusively until the decision is taken to mop up the loose ends. The reward was huge - a fabulous house with 10 garage spaces.

This was a fine incentive to go for a spin, and even though driving within set events would clear up a decent portion of the roads available, the completionist player still had plenty of free driving to do. As the player progressed through the game, they inevitably amassed an incredible car collection. Test Drive Unlimited really did have a great catalogue, which was subsequently improved with DLC. Given that the Project Gotham Series had already introduced the idea of garages and houses for the player to curate their car collection within, Test Drive Unlimited naturally went one better.

Part of the pride I took in my collection was in arranging cars to suit locations and the roads around them. A nifty quick-repeat button for events made grinding a cinch, especially when events like the speed camera tests meant each iteration took less than a minute to complete, so cash was never really a pain to amass. I don't mean that all the events were joyless chores undertaken for a greater good; some events were genuinely fun, such as a long dash solely for Lotuses through heavy traffic, or taking on the giant round-the-island race on one of the much-maligned motorbikes (which handled more like GTA bikes than a serious sim). Notably, the car delivery missions offered huge cash rewards for careful drivers, which often meant gritted teeth and high anxiety if you had to traverse some of Oahu's more treacherous roads.

The Lamborghini Miura - massively overpowered for its class and hence a prime grinding weapon.

The other huge success for Test Drive Unlimited was its famed online integration. It worked pretty much perfectly from launch, and for me it set the stage for a catalogue of amazing moments and serendipitous drives. Multiplayer competition rarely interests me and on Eden's Oahu, it was far more captivating to simply drive around to see who's out there and see what happens when you meet them.

From being chased and hounded by Americans for driving a metallic pink AC 289 - "Is that car really PINK?" - and then outrunning them in the mountains, to meeting up with real-life pals in our latest Ferraris, Zondas and Koenigseggs and heading for the coast, the game's social element really shone. I nearly shed a tear in Oahu's western deserts when driving solo in my offensively sensual black (with gold stripe) Lamborghini Miura.

The roads there tend to have long straights with blind swerves and corners, so you have to be on your toes to keep that Miura's (surprisingly well-recreated) engine note screaming with true Lambo braggadocio. Having been alone for what felt like an hour a bunch of human player labels appeared in the distance, and as their conversations became audible, it turned out to be a group of French and Italian players cruising in vintage cars, chatting in euro-English about how much they f***ing love Miuras. They cheered as I came into view and greeted me as I joined the pack, leaving me feeling more than a little sentimental about the whole event.

It was definitive proof that there's something fundamentally right in the Test Drive Unlimited dream - that simply driving for leisure made perfect sense and that fun and enjoyment doesn't always need structure and artificial constraint to be worthwhile. It proved that the open road and some nice cars is enough for valuable and memorable experiences to emerge naturally, and that with friends, time spent "just" driving around Oahu was often time to be savoured and treasured.

Returning to Test Drive Unlimited today is bittersweet. The servers were turned off long ago and I'd had my glorious end-game save from 2007 corrupted somewhere along the line, but had started anew sometime in 2010. While my collection of cars and houses wasn't as exhaustive, I still had the same starter house and that beloved Esprit V8. Setting the GPS for that northwest lighthouse, I ran what must have been the same route I'd done some seven years ago - and it was just as fun. Cruising at 70 to avoid crashes (and the ire of the omniscient police) has a certain grace and rhythm as you wind through the island's interior highways and its AI traffic.

You could drive to the hills in 2006. And then jump off them on motorbikes.

Coming onto the more rural roads still conveys a sense of making real progress with the journey and, as ever, seeing a clear road ahead and flooring it was a joy just as thrilling now as it was back then. Aside those scrapes with the cops, Test Drive Unlimited is a surprisingly pure experience. It lets you do what you feel and drive where you want without hassling you to do anything else and in doing that, feels more mature and soberly authentic than any of its successors. If anything, it makes you long for a deeper game with more giant locations, more cars and more ambient progression mechanics born out of free-form driving for leisure.

Given the wilder, action-oriented artificialities of Forza Horizon and future open-road prospects, the original Test Drive Unlimited will always stand as a beacon for what's possible when leisure is allowed the same importance as competition. The open-road racer may have not flourished and blossomed in quite the way I hoped it would, but Test Drive Unlimited nonetheless remains an important milestone along a path yet to be properly explored, where things like houses and personalised avatars ratify a more substantial world than we're used to in racing games.

As Eden Studio's greatest game, it's a fitting memorial to revisit, if only to pay respect to forgotten ambitions and the singleminded pursuit of a fabulous concept. In 2006, Test Drive Unlimited felt very much like a generational leap for car simulation genre. As we look forward to the next wave of hardware, it's hard to dismiss Eden Studio's grand project as a genuinely exciting example to follow. I can only hope someone picks up its thread with the same respect for the open road and the joy of driving for driving's sake.