Something unadventurous or inoffensive; opting to go neither one way or the other.

This phrase conjures up similar images to that of 'sitting on the fence', i.e. portraying something that is not sure enough of itself to go one way or the other. It seems a rather odd choice of words to describe something that is bland and safe - surely the middle of the road is the last place to expect safety.

The figurative use of the phrase began in America towards the end of the 19th century. It was used to describe the policies of political parties who attempted to avoid alienating any particular set of voters by saying very little of substance which might annoy them (nothing changes). The term was especially aimed at The Populist Party (or People's) Party. There are early references in print to 'middle of the road' which aren't specifically political in nature but they are considerably outnumbered by those which are. For example, this piece from the November 1891 issue of the Californian magazine Overland Monthly:

"He wanted the people with him. He kept in the middle of the road, and he kept his party there; and he walked in the light and in safety."

Middle of the road is now often used these days to describe bland and undemanding popular music. This is also of US origin and is known there since the 1950s. For example, this piece from the Ohio paper The Lima News, December 1956:

"He [Randy Wood - head of Dot Records] put out some records of local musicians—not country music or downright jazz but what he called "middle of the road music."

By the 1970s this term was well-enough established to spawn its own, if slightly inaccurate, acronym - MOR.