* Agricultural Implements: In many countries that drive on the left (like the UK), the tradition originated from the design of early agricultural tools. People would drive their carts and plows with the reins in their left hand, leaving their right hand free to manage the tools. This made it more practical to keep to the left. This is more of a contributing factor than a definitive cause.
* Military Tactics: Some historians argue that military formations played a part. Keeping to the left (for right-handed soldiers) allowed soldiers to draw their swords with their dominant hand and still have a free rein to control their horses. Again, a plausible contributing factor.
* Early Carriage and Horse-Drawn Vehicles: Similar to agricultural tools, the seating position and control mechanisms on early carriages influenced the habit.
* Napoleon's Influence: Napoleon's armies were instrumental in spreading right-hand driving across much of continental Europe. However, this is more a matter of the consistent adoption of a standard than an original cause. Countries that he conquered adapted to the right-hand drive.
* Industrialization and Mass Production: Once a dominant standard had taken hold in a region, industrialization made it much more difficult to switch sides. Changing the side of the road would require significant and costly changes to infrastructure.
It's important to note that there isn't one single definitive answer. The left-hand vs. right-hand driving convention developed organically over centuries, driven by a combination of these historical factors and gradual standardization within regions. It's a matter of historical contingency; once a standard was established, it became extremely difficult to change.
