Under the CDL manual, when can a bus driver approach a drawbridge without coming to a full stop, while still slowing down and checking that it's safe?

49 CFR 383.117(a)(4), proper procedures at railroad-highway grade crossings and drawbridges; AAMVA CDL Manual Section 4.3.6, p. 4-2 to 4-3

These two exceptions exist because a green light or a human traffic controller already confirms it's safe to proceed, which is the same underlying safety need the full stop otherwise serves. Knowing the specific exceptions prevents drivers from either stopping unnecessarily or skipping a stop when neither exception actually applies.