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The A24 motorway towers over the Douro River in northern-central Spain, with the N2 below.douglas Firby/The Globe and Mail

For many driving enthusiasts, there is no such thing as the perfect road. What do they know? Science has found something very close to perfection in a 27-kilometre stretch of two-lane excitement in Portugal’s Douro Valley.

Route N-222 cuts through the heart of the country’s famous Douro wine-producing region, from Peso da Regua east to Pinhao. This short drive has everything a driver craves, including 93 unpredictable bends, speedy straights and jaw-dropping scenery as it tracks the southern bank of the winding Douro River.

There are, of course, hundreds of exciting roads around the world. In some cases, it feels as though the mischievous spirits who plotted these serpentine trails had heart-stopping thrills in mind. None, however, has the elements as beautifully balanced as N-222. Science has proven it.

Yeah, that’s right, there’s a formula for perfection. In this case, it’s called the Avis Driving Ratio, or ADR. For that, you can thank Mark Hadley, PhD, a quantum physicist at Britain’s University of Warwick.

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There are stone walls where there should be shoulders and speed limits that are largely ignored by drivers, as we zip past the turnoff for Quinta Das Caldas winery.douglas Firby/The Globe and Mail

A few years ago, Avis, the rental company, was looking for a way to get people excited about renting sporty cars. The company wanted to get drivers out of Puntos and into Porches. So, they began the search for not only the very best road, but also great roads around the world. (See the Top 25 list).

Finding that One Great Road is this big ol’ world is tougher than finding a Lada that doesn’t belch blue smoke. The task is quixotic, yet three experts embraced the challenge: Hermann Tilke, a Formula 1 track designer and retired racing driver; John Wardley, a roller-coaster designer; and the scientist, Hadley. Undaunted by the odds, the team identified four criteria for a great drive: bends, acceleration, cruising and braking. The best roads strike a balance between the adrenaline rush of speed and acceleration, the skill of cornering and short stretches of straight that allow the driver to enjoy the scenery.

Good driving is all about the rush. To understand the emotional thrill, Hadley spent a track day at Rockingham Motor Speedway with Tilke, and then visited Britain’s Alton Towers amusement park with Wardley. The team concluded that the ideal driving ratio is 10:1 – 10 seconds on a straight with one second spent on a bend. With the concept in hand, they started sifting through recommended roads from around the world, mapping each against the parameters. Not on pavement, but on paper.

“I got my protractor and starting moving around,” Hadley said in a telephone interview from his home in Warwick. “It was all done from a map.”

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Terraced hillsides where old vines grow on the north bank of the Douro River constantly compete for the driver’s attention.douglas Firby/The Globe and Mail

In fact, Hadley has never driven the road he identified as the best. He’s not much of a sporty car driver. He can’t even say how many of the Top 25 roads he has driven, although he’s sure he’s been on a few.

We did. And the experience was not what we expected. The full Route N-222 is actually much longer than the award-winning stretch. It starts in the Atlantic port city of Porto, and wends south and east through hilly terrain before it arrives two hours later at the town of Peso da Regua. It is a thrilling, stomach-churning and exhausting drive through tiny villages that conceal surprises at every corner. The area past Pinhao is similar – climbing up steep hills as it twists its way toward the eastern edge of the country.

Those sections, while exhilarating, do poorly on the ADR because the emphasis is almost entirely on cornering. (Thankfully, there is an easier drive, via a motorway.) The stretch of N-222 from Peso da Regua east to Pinhao, by comparison, is walk in the park – unless you push well beyond posted speed limits, which vary from 50 km/h to 100 km/h.

There are no steep climbs up hills near precipitous cliffs and no single-lane squeeze points. There is, however, a beautiful blend of driving experience and the scenery of terraced vineyards rising above the gently flowing river. And, just to add a touch of adrenaline, there are sections where you zip within a metre of concrete retaining walls as you navigate bends with unpredictable radii – sometimes way tighter than you expect. You have to be sharp at every turn.

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Route N-222 cuts through the heart of the country's famous Douro wine-producing region, from Peso da RŽgua east to Pinho.douglas Firby/The Globe and Mail

Heading east from Peso da Regua, terraced hills of vines rise above the Douro River, basking gold and red in the late-October sun. The road sits practically on the edge of the river, as you pass winery after winery, roll past the Barragem da Regua dam, through the hamlet of Cais De Baugauste, across the Remilobos River, before stopping for wine and lunch at the Quinta de Tedo, where the Tedo River flows into the Douro.

The tightest bends are near the eastern end of the drive as you navigate a hairpin just before landing in Pinhao. You can drive the entire stretch in less than half an hour – much less, if you dare.

It is a road that virtually cries out for you to break the law, which it appears most local drivers do. But Hadley insists the idea is not to speed – the thrill is in the scenery, acceleration, braking and navigation.

Hadley knows the Douro’s beauty, because he has been through the valley on a train that traces the north shore of the river. He admits he would like to drive it, but in a sporty car, not his normal touring vehicle, a 25-year-old Iveco truck – a five-tonne monstrosity built and sold in Europe.

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This short drive has everything a driver craves, including 93 unpredictable bends, speedy straights and jaw-dropping scenery as it tracks the southern bank of the winding Douro River.douglas Firby/The Globe and Mail

Driving enthusiasts also tend to be adrenaline-junkies. For them, the “world’s best road” may prove to be a disappointment. It hasn’t been for Avis, which saw a jump in sport-car rentals after it announced this road in 2015.

It is, however, curious that not one of Canada’s great driving roads made the shortlist – especially when the tedious Overseas Highway from Long Point Key to Florida Keys snuck in at number 25. Hadley says that road’s unique scenery earned it a place on the list.

But whither the Cabot Trail? The Fundy Coastal Drive? Alberta’s Icefield Parkway? The Sea to Sky Highway? It appears they will remain our secret treasures.

Favourite roads

It might come as a shock to Canadians that not one of our great driving roads made it into the Top 25 of this list. Here are the Avis’ experts picks for the world’s best driving roads, using an ideal ratio of 10:1 -- that is, 10 seconds on a straight compared with one second spent on a bend.

1. Portugal: N-222 Peso da Régua to Pinhão

Avis driving ratio (ADR): 11.3:1

2. California: Highway 1 from Castroville to Big Sur, California

ADR: 8.5:1

3. UK: A535 from Holmes Chapel to Alderley Edge

ADR: 8.5:1

4. Switzerland: Flüela Pass from Davos to Susch

ADR: 7.7:1

5. UK: B3515 from Cheddar to Ashwick

ADR: 7.5:1

6. Germany: Titisee to Bad Krozingen via L126

ADR: 7.1:1

7. UK: A591 from Kendal to Keswick

ADR: 14.3:1

8. Australia: Great Ocean Road from The Twelve Apostles to Apollo Bay

ADR: 5.2:1

9. France: Route de Napoleon

ADR: 15.1:1

10. New Zealand: The Karamea Highway

ADR: 5.0:1

11. Spain: San José (Almería) to Cabo de Gata via the ALP-822

ADR: 5.0:1

12. Germany: S165 from Hohnstein to Bad Schandau

ADR: 3.8:1

13. Spain: Zumaia to Zarauz via the N-634

ADR: 3.7:1

14. Portugal: N247/Avenida do Atlântico, Sintra to Praia das Maças

ADR: 2.6:1

15. Spain: Escalona to Rios Bellosvia the HU-631

ADR: 2.2:1

16. Portugal: N267 São Marcos da Serra to Monchique

ADR: 2.1:1

17. Japan: Nihon Romantic Highway from Chugushi to the 120 road split

ADR: 2.0:1

18. France: D81 from Piana to Porto in Corsica

ADR: 2.0:1

19. Italy: Portofino (GE) to Portofino Vetta (GE)

ADR: 1.9:1

20. Romania: The Transfăgărășan aka the DN7C

ADR: 1.9:1

21. Norway: The Lysebotn Road

ADR: 1.7:1

22. Italy: The Amalfi Coast route

ADR: 1.3:1

23. North Carolina: Tail of the Dragon

ADR: 1.1:1

24. Argentina: Ruta 40 from Cuesta de Miranda to the La Rioja/San Juan province borderline

ADR: 23.7:1

25. Florida: Overseas Highway from Long Point Key

ADR: 240:1

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