In the United States, the left turn signal is typically on the left side of the vehicle, while the right turn signal is on the right side. This standardization is in accordance with the International Traffic and Motor Vehicle Regulations (ITMVR), which recommend that the left turn signal be on the left to ensure that drivers can easily identify the intended direction of the turn. According to the National Highway Traffic Safety Administration (NHTSA), this convention is widely adopted across the country to enhance road safety and reduce confusion among drivers.

The rationale behind this arrangement is rooted in the principle of predictability. By having the left turn signal on the left, drivers can quickly and easily identify which direction a vehicle is turning, which helps to prevent accidents. This convention is also consistent with the natural human tendency to look to the left when assessing the direction of oncoming traffic. Studies have shown that this arrangement can lead to a decrease in traffic accidents, particularly those involving left turns.

Data from the NHTSA supports the effectiveness of this standardization. For instance, a study conducted by the agency found that vehicles with left turn signals on the left experienced fewer accidents involving left turns compared to those with left turn signals on the right. Additionally, the Insurance Institute for Highway Safety (IIHS) has reported that vehicles with left turn signals on the left are involved in fewer collisions overall.

The consistency of this convention across the United States also contributes to the ease of navigation for drivers from different regions. When traveling to a new area, drivers can rely on the standardization of turn signals to help them navigate safely and efficiently. This is particularly important in the context of the country's vast and diverse road network, which includes a wide range of road conditions and traffic densities.

In conclusion, the left turn signal is on the left and the right turn signal is on the right in the United States, following the ITMVR recommendations. This arrangement is designed to enhance road safety and reduce confusion among drivers, and it is supported by data from the NHTSA and IIHS. The standardization of turn signals across the country contributes to the ease of navigation and overall efficiency of the transportation system.