History of which side to drive. (but note I beeves in all countries people mount and dismount horse from the left i.e. footpath side in UK)

Wikipedia:-

In 1998, archaeologists found a well-preserved track leading to a Roman quarry near Swindon, England. The grooves in the road on the left side (viewed facing down the track away from the quarry) were much deeper than those on the right side. These grooves suggest that the Romans drove on the left, at least in this particular location, since carts would exit the quarry heavily loaded, and enter it empty[4].

In fact, some, for example C. Northcote Parkinson, believed that ancient travellers on horseback generally rode on the left side of the road. As more people are right-handed, a horseman would thus be able to hold the reins with his left hand and keep his right hand free—to offer in friendship to passing riders or to defend himself with a sword, if necessary.[26]

The first legal reference in Britain to an order for traffic to remain on the left was in 1756 with regard to London Bridge. The Highway Act 1773 contained a recommendation that horse traffic should remain on the left and this is enshrined in the Highway Act 1835.[27]

In the late 1700s, the shift from left to right that took place in countries such as the United States was based on teamsters' use of large freight wagons pulled by several pairs of horses. The wagons had no driver’s seat, so a postilion sat on the left rear horse and held his whip in his right hand. Seated on the left, the driver preferred that other wagons pass him on the left so that he could be sure to keep clear of the wheels of oncoming wagons. He did that by driving on the right side of the road.[26]

Countries that became part of the British Empire adopted the British keep-left rule, although many have since changed. In Canada, the Maritime provinces and British Columbia initially drove on the left, but changed to the right in order to make border crossings to and from other provinces easier. Nova Scotia switched to driving on the right on 15 April 1923.