It is important to distinguish ADVERB (one of the four major parts of speech) from ADVERBIAL, one of the five functions of the phrase in the clause.
The A slot can be filled by AvP, PP or NP.
This sentence illustrates the fact that a clause may contain several A's. Many of these are MOBILE constituents which can change position without making the clause ungrammatical. Moving A may, however, produce subtle variations in meaning, as in (16), where there is more emphasis on the time when they travelled.
16. Last year, however, they travelled regularly on the 59 bus.