Trucks in Japan mystery

Back in the 90s, trucks driving on expressways in Japan had three(?) green lights above the driver's cabin that might all be lit, or else just one or two lit.

I could never work out what they were for.

Today when driving on the expressway I noticed that trucks don't seem to have those lights any more.

What were they??
When and why did they disappear?!

by koyanostranger