Why is it that people who use English as a second language use the word "highway" to mean a motorway or freeway.?

Because it's not just people who use English as a second language who do this - this seems especially prevalent in Canada and the Northeast US. ("Then I got on the highway and stayed on it for 3 exits.")

A lot of people who use English as a second language, people in Belgium for instance who generally speak English very well, use the word highway to describe what we call motorways in Britain and what Americans call a freeway or interstate. Google maps and GPS devices also seem to use this terminology. The Oxford English Reference Dictionary says that a highway is just a public road. In America people talk about US Highway 41 for example which is called this even when it is a two lane road. So how did highway come to mean a motorway or freeway?

The word "highway" is much older than "motorway" (British, mid 1950s) or "freeway" (American, maybe 1930s). You find the term "highway" in the King James Bible of 1611, and a well known expression is "the Queen's highway".

As an older Briton, born before motorways, I would say "main road" in general. For a road improved and somewhat straightened in the early days of motoring, maybe 1920s or 1930s, the official term is "trunk road" for a long distance road. A "bypass" (usually dual carriageway, i.e. with a central divider) takes traffic around the centre of a large town, to avoid mid-city congestion.

Any more questions about road types?

Possibly because the American usage describing a motorway or freeway is generically "highway".

In America you will NEVER hear us saying the word "motorway". 50 years ago, there would be freeways (as opposed to toll roads) but since the establishment of our interstate highway program, we have interstates and highways and toll roads.

Only those who learned American English say highway. In the UK a highway is any public road whether it's a country track or a motorway.