When Sweden changed driving side, that is exactly what happened.
"The relatively smooth changeover saw a reduction in the number of accidents. On the day of the change, only 157 minor accidents were reported, of which only 32 involved personal injuries, with only a handful serious. On the Monday following Dagen H, there were 125 reported traffic accidents, compared to a range of 130 to 198 for previous Mondays, none of them fatal. Experts suggested that changing to driving on the right reduced accidents while overtaking, as people already drove left-hand drive vehicles, thereby having a better view of the road ahead; additionally, the change made a marked surge in perceived risk that exceeded the target level and thus was followed by very cautious behavior that caused a major decrease in road fatalities. Indeed, fatal car-to-car and car-to-pedestrian accidents dropped sharply as a result, and the number of motor insurance claims went down by 40%.
These initial improvements did not last, however. The number of motor insurance claims returned to 'normal' over the next six weeks and, by 1969, the accident rates were back to the levels seen before the change." https://en.wikipedia.org/wiki/Dagen_H