הנה הסבר מויקיפדיה
Japan is one of the few countries outside the former British Empire to drive on the left. An informal practice of left-hand passage dates at least to the Edo period, when samurai are said to have passed each other to the left in order to avoid knocking swords with each other (as swords were always worn to the left side). During the late 1800s, Japan built its first railways with British technical assistance, and double-tracked railways adopted the British practice of running on the left. However, army troops were ordered to keep to the right while travelling on roads, creating a double standard that was not legally resolved until 1924, when all road travel in Japan switched to the left. ובכן מה שאמרתי לא היה מדוייק. מה שהוקם בסיוע של הבריטים זה מערכת הרכבות, שכן אז אנגליה היתה המובילה הבלתי מעורערת בתחום הזה, ובגלל זה הרכבות נסעו בצד שמאל. משם המנהג אומץ גם כשפיתחו מערכת של כבישים יותר מאוחר.