Entering a Japanese Expressway: Acceleration Lane and Minimum Speed

When entering an expressway in the acceleration lane, what should a driver do?

Use the acceleration lane, build your speed up to match the traffic on the main carriageway, signal, and merge without making anyone already there slow down or swerve. Each of those three parts is a separate legal duty, and the test checks them separately.

First, using the lane is not optional. Art. 75-7 of the Road Traffic Act states that a vehicle about to enter the main carriageway must travel along the acceleration lane where one is provided (must use it). Second, the Rules of the Road require you to accelerate sufficiently in it (accelerate enough) — the point of the lane is to erase the speed difference before you arrive, not after. Third, and the single most commonly failed statement in this cluster: a vehicle entering the main carriageway does not have priority over vehicles already on it, and must not obstruct their progress (no priority when merging). The same non-obstruction duty applies where two main carriageways join and a marking designates the one ahead as the priority carriageway.

The mirror image of the rule applies on the way out: where a deceleration lane (減速車線) is provided you must use that too — it is not the driver's free choice — and you should move into the lane connecting to your exit in advance and slow down while watching the speedometer rather than trusting your feel for speed.

What exactly is the "main carriageway" (honsen-shadō)?

It is the part of the expressway where traffic normally runs at speed — and it deliberately excludes the acceleration lane, deceleration lane, climbing lane, roadside strip and shoulder. The Road Traffic Act defines it as the carriageway formed by the main lanes of a national expressway (高速自動車国道) or a motor-vehicle-only road (自動車専用道路), in Art. 2(3-2) (definition), and the Rules of the Road spell out the exclusions (what is not included).

This is not a vocabulary trivia point — it is what makes the rest of the rules readable. The speed table, the ban on U-turns and reversing, and the acceleration-lane duty are all written about the main carriageway, so knowing where it starts tells you when each rule switches on. It also explains why the acceleration lane is treated as a place to prepare rather than a place to drive: you are not on the main carriageway yet.

What is the minimum speed on a Japanese expressway?

50 km/h on the main carriageway of a national expressway, where no sign or marking specifies otherwise. The Rules of the Road give the figure in the same table that sets the maximum speeds, and both edges of the table are binding: you may not exceed the maximum, and you may not drive slower than the minimum (minimum applies too).

Two different articles create the duty, which is worth separating because the test uses both. Art. 23 covers roads where a minimum speed is posted by a sign (posted minimum), and Art. 75-4 covers the main carriageway of a national expressway, where the minimum applies from the statute itself rather than from a sign. On the maximum side, the Enforcement Order sets 100 km/h for ordinary cars (100 km/h), 90 km/h for large trucks and similar goods vehicles (90 km/h), and 80 km/h for everything outside those two groups, such as three-wheeled vehicles and cars towing a trailer (80 km/h). One further trap: on a section where the two directions are not structurally divided, the expressway table does not apply at all — ordinary-road rules do (undivided section).

Is it ever recommended to drive below the minimum posted speed?

Yes — and the law says so directly, in the same sentence that creates the duty. Both Art. 23 and Art. 75-4 apply except where the law requires a reduction in speed, and except where slowing down is unavoidable in order to prevent danger.

So statements built on the word 'never' are wrong by construction. 'Below the minimum is never allowed, not even to prevent danger' is false on ordinary roads and false on expressway main carriageways alike, while 'you do not have to keep to the minimum where it is unavoidable to prevent danger' is true. In plain terms: congestion, an accident ahead, thick fog, a place where the law tells you to slow down — in all of these, safety outranks the minimum speed. What the minimum speed forbids is choosing to crawl on a clear road, because a large speed difference is itself a hazard on an expressway.

What does the minimum speed sign look like, and what does it mean?

It is a white disc with a red rim and blue numerals — and the number has a line drawn underneath it. That underline is the whole difference between it and the maximum speed sign. The sign is No. 324 in the Sign Order, and it designates a minimum speed under Art. 23 or Art. 75-4 (what it designates, the underline). The maximum speed sign, No. 323, is the same white disc with the same red rim and the same blue numerals — just without the line — which is exactly why the pair is worth ten seconds of your study time.

Because this sign sets a floor and not a ceiling, three of the four ways the test can misread it are all false. You may travel faster than the figure shown, within the applicable maximum. You must not read it as a maximum (not a maximum). And you are not required to hold exactly that speed. Where traffic is congested and the speed simply cannot be reached, the duty gives way for the reasons in the section above. The reverse misreading is tested too: a posted maximum never doubles as a minimum (not a minimum). For how the sign families work in general, see the guide on road signs on the written test.

Can I stay in the left lane, and where else am I allowed to drive?

Yes — the left lane is where you are supposed to be. Japan drives on the left, and the default is the first lane counting from the left edge. With two lanes in the same direction you must use the left one; with three or more, keep the rightmost lane free for overtaking and use the others (leave the rightmost free). Within your lane, the Rules of the Road suggest positioning slightly to the left, using the white line on your left as a guide — not slightly right.

What you may not do: drive on the roadside strip or the shoulder, even in congestion (never for driving) — although stopping there is permitted when a breakdown leaves you no choice and the shoulder is wide enough. You also may not make a U-turn, reverse, or cross the central reservation on the main carriageway (all three banned). And the climbing lane (登坂車線) is for slow vehicles such as loaded trucks, not a passing lane for fast ones (not for overtaking).

Who may not use an expressway at all?

Learner drivers, slow vehicles, and small-engine two-wheelers. If you hold a provisional (learner's) licence — 仮免許 karimen — practice driving may not be done on a national expressway or a motor-vehicle-only road; the Enforcement Regulations restrict learner practice to other roads (no learner practice on expressways). That is a rule about practice with a karimen, not about newly licensed drivers, who may use expressways normally.

The other exclusions follow from the speed difference. Mini-cars, small motorcycles of 125 cc or less and general mopeds may not use expressways at all (which vehicles are barred). Nor may a vehicle that structurally cannot reach 50 km/h, such as a farm work vehicle — note the threshold is 50, not 60 — or a vehicle that cannot maintain 50 km/h because it is towing another vehicle. Read together with the 50 km/h minimum, the pattern is consistent: an expressway assumes every vehicle on it can hold 50 km/h or more.

FAQ

When entering an expressway from the acceleration lane, what should you do?

Use the acceleration lane — it is mandatory where one is provided — accelerate sufficiently to match the speed of traffic on the main carriageway, signal, and merge without obstructing vehicles already on it. A merging vehicle has no priority over traffic already on the main carriageway.

What is the acceleration lane on expressways used for?

It is used to build your speed up to the speed of the traffic on the main carriageway before you merge, so that the speed difference is gone by the time you join. Road Traffic Act Art. 75-7 makes using it compulsory where one is provided.

What is the minimum speed on a Japanese expressway?

50 km/h on the main carriageway of a national expressway where no sign or marking specifies otherwise. Where a minimum speed sign is posted, the figure on the sign applies instead.

Is it ever recommended that you drive below the minimum posted speed limit?

Yes. Both Road Traffic Act Art. 23 and Art. 75-4 except cases where the law requires you to reduce speed and cases where slowing below the minimum is unavoidable in order to prevent danger. Safety takes precedence over the minimum speed.

What does the minimum speed sign mean?

It designates a minimum speed: you must not travel slower than the figure shown. You may travel faster, up to the applicable maximum, and you are not required to hold that exact speed. You can tell it from the maximum speed sign by the line drawn under the number.

Can you drive on an expressway with a student or provisional licence?

No. Practice driving under a provisional (karimen) licence may not be carried out on national expressways or motor-vehicle-only roads. Once you hold a full licence there is no such restriction.

Can you stay in the left lane on an expressway?

Yes. The left lane is the default: with two lanes in the same direction you must use the left one, and with three or more you keep the rightmost lane free for overtaking. Within the lane, position yourself slightly to the left.