National Route 16
Major commercial Traffic route. OK to ride in short sections, but expect trucks to hammer past you with no space, and grim air quality especially in summer.
Major cities served by R16 include Yokosuka, Sagamihara, Hachioji, Kawagoe, Saitama, Kasukabe, Kashiwa, Chiba and Kisarazu. It passes in a loop clockwise through all these, some 241 km, before turning into a ferry to cross back from Chiba pref. to Kanagawa pref. and continue its circle of diesel fumes.
In several locations e.g. near Hachioji the road splits into two with both the original road and a newer parallel road ("Hachioji Bypass") being labelled R16.
Possibly one of the worst imaginable bike rides would be to ride the entirety of R16 on a Monday.
National Route 246
Known variously as Aoyama-dōri, Tamagawa-dōri, Atsugi-kaidō, Route 246 runs 122 km from Miyakezaka junction on the Tokyo Imperial Palace loop to Numazu in Shizuoka prefecture. R246 is a busy road, but offers an advantage for cycle commuters over similar arterial roads in that during morning (inbound 07.00 - 09.30) and evening (outbound 17.00 - 19.00) rush hours the leftmost Traffic lane is reserved for buses, bicycles and scooters between Shibuya and Komazawa-daigaku. Police occasionally enforce this, and apart from a few bad apples, notably in BMW, Mercedes, Audi, taxis etc., most drivers observe this rule.
R246 passes through Futago-tamagawa - an ideal spot to get on the Tamagawa cycle path, and also passes through Hadano for the start of the Yabitsu-tōge climb.
Saitama Prefectural route 2
From Arakawa to Kasukabe. Busy road, but direct route from north Saitama down to the Arakawa through Omiya. Surface is OK, with no rough sections. Goes through Route 16 a few times, which slows things down and brings you in contact with trucks again.