A street road is as wide as a street plus a road combined together. So cool! So wide! It's a highway!
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. It uses the かい reading (not the がい) because it's at the beginning of the word.
Tokai-do Road, Nakasen-do Road, Nikko-kaido Road, Koshu-kaido Road, and Oshu-kaido Road are called Gokaido, which were the five principal routes used during the Edo period.