One of the tests that you can use in this case is to observe the verb and see if it can be pluralized together with the subject. In a "passive se" sentence you can do it; in an impersonal sentence it's impossible (i.e. it produces an ungrammatical sentence). The verb in impersonal sentences is always in the third person singular.
If instead of a new highway you were referring to new houses, for example, you would say:
El martes pasado se inauguraron las nuevas casas.
This sentence has the same structure as your example, only the verb is in the plural to agree with the new subject. Since it's a correct grammatical sentence, then it must be a passive, and so must also your example.