What is the main purpose of the outermost lane on an expressway with multiple lanes in one direction?

The primary purpose of the outermost lane on an expressway is designated for slower moving vehicles. This lane is typically used by vehicles that do not need to travel at the maximum speed of other lanes, providing a safe space for them to operate without interfering with faster traffic.

By having a dedicated lane for slower vehicles, expressways can enhance overall traffic flow and safety, allowing faster vehicles to pass unimpeded in the adjacent lanes. This arrangement helps reduce the chances of accidents caused by slower vehicles merging into faster-moving traffic unexpectedly.

In contrast, the other options, while valid in some contexts, do not accurately reflect the main function of the outermost lane. Heavy trucks, while often found in outer lanes on certain roads, do not specifically denote the main purpose of that lane as they may utilize multiple lanes depending on their speed and size. The outermost lane is generally not intended primarily for overtaking; it is rather focused on accommodating those traveling at lower speeds.