Why is the Audi A2 considered a pioneering car in terms of its construction and design?

I've heard that the Audi A2 was ahead of its time when it comes to its construction and design. Can you tell me more about it?

The Audi A2 was one of the first mass-produced cars to feature an aluminum space frame which significantly reduced its weight and improved fuel efficiency. It was also a pioneer in terms of aerodynamic design, with a drag coefficient of just 0.25. Overall, it set new standards for lightweight and energy-efficient car production.