Two-Up on the Tomei: Age, Licence History and a Marginal Machine
Kenji is 20 years and 4 months old and holds a 大型自動二輪免許 obtained six months ago. Before that he held a 普通自動二輪免許, taken at age 17 years 10 months, so his motorcycle licence history is 2 years 6 months, not 3. His bike is a 1,300 cc naked model, 2019, 21,000 km, fitted with a top box. He plans a Saturday 22:00 departure from Tokyo to Shizuoka: 180 km of 東名高速, arriving around 00:30. The forecast gives a 60% chance of rain after midnight. The pillion would be a 20-year-old friend who has never ridden a motorcycle and owns no helmet. Rear tyre tread depth is 1.7 mm, cold tyre pressure 190 kPa against a 250 kPa specification, and chain slack is 44 mm against a 25–35 mm specification. He intends to pass the toll gate on ETC and to sit in the left lane at 80–90 km/h.
1. Is the planned two-up expressway run lawful as described? Separate the elements that are illegal from those that are legal but unwise.
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Two conditions must both be satisfied before a 自動二輪車 may carry a passenger: the rider must be at least 20 years old, and 3 years must have elapsed since obtaining a 大型自動二輪 or 普通自動二輪 licence. Kenji meets the age condition but his 二輪 history is only 2 years 6 months, so carrying any pillion is unlawful - on ordinary roads as much as on the 東名. The expressway is not the obstacle: two-up riding on expressways has been permitted for 大型二輪 since 2005, provided those two conditions are met.
Outright illegal: the pillion without a helmet (a helmet is mandatory for every rider and passenger), and the two-up ride itself. The passenger must also sit on the proper pillion seat with both feet on the footrests, within the 乗車定員.
Lawful but poor judgment: 80–90 km/h at night with rain forecast, a first-time passenger who cannot read the road, a rear tyre at 1.7 mm (barely above the 1.6 mm minimum) and 60 kPa under-inflated, plus an over-slack chain. Each error compounds the others; the correct call is to fix the machine and drop the pillion plan, not merely to slow down.
2. Assume the pillion issue is resolved (an experienced adult with her own helmet). Rank the mechanical findings and decide whether the 180 km night run should proceed.
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Rank by consequence, not by how easy the fix is.
- Chain slack 44 mm against 25–35 mm. Over-slack lets the chain whip, hook the sprocket teeth or jump off under acceleration; a derailed chain can punch through the crankcase. This is a 10-minute adjust with the right tools and must be done cold, at the tightest point.
- Rear tyre 190 kPa against 250 kPa. That is roughly 24% under-inflation. On a heavy 1,300 cc machine carrying two people, a soft tyre flexes, heats, wears the sidewall and badly degrades wet grip. It may also indicate a slow leak that should be found, not simply pumped up.
- Tread 1.7 mm. Legal, but with two-up load, rain and 1.7 mm there is very little water-clearance margin left; replace before any wet long run.
Go/no-go: after inflation, chain adjustment and a load-rating check for two-up, a dry daytime run is defensible. For a 22:00 start with 60% rain probability, a first long two-up trip and an already marginal rear tyre, the answer is postpone. Fatigue plus darkness plus rain reduces the safety margin faster than any of the items can be fixed at a service station.
3. It is now 00:40. Rain is heavy, he is running at 100 km/h and a truck ahead is throwing up spray. Set out the decision sequence for the next few minutes.
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The sequence should be: reduce, separate, reposition, then decide whether to continue at all.
Reduce. Ease off the throttle and let speed fall on a closed throttle rather than braking hard on a wet surface. In heavy spray, effective visibility may be 50 m or less, so the safe speed is the one that lets him stop within what he can see - well below the posted limit. Separate. Increase the following distance to at least 4 seconds. Spray is not just discomfort: it hides the road surface, and the truck's wake pushes a light two-wheeler around. Reposition. Stay in the left (first) lane, but move out of the direct spray cone; avoid straddling lane markings and painted arrows, which are slippery when wet, and avoid the drainage grates at the left edge. Smooth inputs. Both brakes progressively, no sudden steering; ABS helps but does not raise the tyre's grip on paint or standing water. Decide. Stopping on the carriageway is prohibited and lethal; the only safe stops are an SA/PA or a 非常駐車帯. At 00:40 with heavy rain and six hours awake, exiting at the next SA to wait out the rain is the correct judgement, not a failure of nerve.