Door hinges are designed for left and right to accommodate the directional flow of traffic and ensure the door opens and closes smoothly. This design is based on the standard left-hand drive (LHD) and right-hand drive (RHD) configurations of vehicles and pedestrians in different regions of the world.

The primary reason for this design is to align with the prevalent traffic flow in a specific area. In countries where vehicles drive on the left side of the road, such as the United Kingdom, Australia, and India, doors are typically designed with hinges on the left side to facilitate the opening of doors without obstructing oncoming traffic. Conversely, in countries where vehicles drive on the right side of the road, such as the United States, Canada, and most European countries, doors are designed with hinges on the right side for the same reason.

Data from the United Nations Economic Commission for Europe (UNECE) indicates that as of 2019, approximately 65% of the world's population lives in countries with right-hand traffic, while about 35% live in left-hand traffic countries. This imbalance necessitates the design of door hinges to cater to both traffic configurations. For instance, in the United States, where right-hand traffic is the norm, the majority of vehicles and buildings are equipped with right-hand hinges, while in countries like the United Kingdom, left-hand hinges are standard.

The design of door hinges for left and right also takes into account the natural human tendency to open doors with the hand opposite to the hinge side. This practice is known as "handedness" and is influenced by factors such as cultural norms, personal preference, and ergonomic considerations. Studies have shown that individuals tend to open doors with their dominant hand, which is usually the right hand for right-handed people and the left hand for left-handed people. By aligning door hinges with the dominant hand, the door can be opened more easily and with less effort.

In conclusion, door hinges are designed for left and right to accommodate the directional flow of traffic, ensure smooth operation, and cater to the natural handedness of individuals. This design is based on the global prevalence of left-hand and right-hand traffic and the ergonomic preferences of door users.