From Daniel Faigin's amazing California Highways website:
1. US 101. 807 miles
2. I-5. 796 miles
3. CA 1. 656 miles
4. US 395. 557 miles
5. CA 99. 415 miles
6. CA 299. 307 miles
7. CA 65. 305 miles
8. CA 49. 295 miles
9. I-15. 294 miles
10. CA 33. 290 miles
Here in Toronto, Ontario, Yonge Street is "At 1,178 miles… it's touted as the world's longest street."
And Highway 401 is over 814.4 km (based on 2000 stats.)
Just think of how long it took to build these roads. Wow!
I recently drove the entirety of Highway 101 and I have to say it is one of the most beautiful and fascinating stretches of road in the world.