Short answer: from 18:00 to 06:00, and whenever fog, smoke, dust, rain or bad weather cuts visibility, a motorbike’s lights must be on. Riding unlit in those conditions is a violation under Article 7(1)(g) of Decree 168/2024/NĐ-CP at 200,000–400,000 VND. Tunnels have their own rule — dipped beams inside, at 600,000–800,000 VND — and a broken or missing headlight is an equipment defect at 400,000–600,000 VND before the light-use rule is even reached.
The window the rule names
The decree defines the duty by clock and by weather: lights from 18:00 the previous evening to 06:00, or in fog, smoke, dust, rain or any weather that limits sight. Two things follow. First, the clock rule is absolute in both directions — a rider at 17:58 in good light has not violated anything, and a rider at 18:05 in bright city glow has. Second, the weather rule works outside the window: a grey Hanoi drizzle at four in the afternoon with visibility down is a lights-on condition under the same clause.
Hanoi’s actual dusk lands between the two clock edges for most of the year, and the street-lighting argument does not apply: the rule is the rule, not a brightness test.
High beam and tunnels
The same low band prices beam misuse: using high beam against oncoming traffic, against pedestrians crossing, or on lit-up stretches through residential areas is a violation in the 200,000–400,000 VND family. Inside road tunnels the requirement is explicit — ride on dipped beam — and failing that sits in the 600,000–800,000 VND band under Article 7(3)(h). A rider who enters a road tunnel on high beam is both dazzling the tunnel and violating the tunnel rule at the heavier price.
When the light itself is the defect
The light-use rules assume a working light. A bike with no headlight at all, a dead one, or one below the design standard is an equipment violation under Article 14(1)(c) — 400,000–600,000 VND — and the bike should not be ridden at night at all. The bands stack by logic: an unlit rider at night with a dead headlight is one stop producing both findings, the defect and the failure to light.
Why this is the fine riders should want enforced
An unlit motorbike at night is invisible until it is unavoidable — Vietnamese urban traffic is dense, unlit couriers ride the kerb lanes, and the closing geometry of a dark bike against oncoming headlights kills. For renters the check is one switch-flick at handover and one glance at dusk. For everyone else: lights on at the clock edge, dipped beam in traffic, and no excuses about battery drain that a five-minute charge routine does not solve.
Information, not legal advice. Verified against Decree 168/2024/NĐ-CP and Law 36/2024/QH15 on 2026-09-22.