I didn't read this as applying to 'on ramps' (we call them sliproads in the UK) that are lanes merging onto a highway/freeway/motorway. It wouldn't make sense under any circumstances for people on the highway to slow down and stop to let traffic join! I think this is referring to two 'equal' lanes of one road that become one — and, thus, merge. I don't know if they're unusual in other countries, but they're definitely a thing in the UK.