During the 18th Century, Toll Roads Began Appearing All Over

During the 18th century, toll roads began appearing all over England, spreading from London all the way to Scotland. But these roads were made of small stones piled on top of mud, so even a light rain could render them impassable. A blind engineer named John Metcalf came up with a solution. He insisted that roads have a strong stone foundation. He "crowned" the roads too, meaning that he made them higher in the middle than on the sides. He added a ditch on either side, providing proper drainage to keep the roadways from caving in due to excess moisture. This also prevented potholes. But his methods proved costly and many local governments were loath to pay the construction costs. Undeterred, Metcalf challenged a colonel to race him to London. Thanks to the lousy road conditions, he made it to the city on foot faster than the colonel could get there in his horse-drawn buggy. As a result, towns across England began adopting Metcalf's engineering ideas. Even today, many of these ideas are still in use not only in England but around the world.
What is the main idea of the passage?

A) Metcalf's road-building methods proved costly.
B) Metcalf raced a man in a horse and buggy to London and won.
C) Metcalf was blinded at a construction site.
D) Metcalf introduced modern road-building techniques.