Il a descendu le boulevard st michel

Jim's right. For French people, going down a road is the same as climbing down a ladder (NOT getting off a ladder, though). Getting off a plane is different in English as well, as, in both languages, it takes an indirect object rather than a direct one.

I a descendu les escaliers. -- He went down the stairs. "Les escaliers" is a direct object. You can see that by trying to squeeze in a preposition in English. If you can't easily do it without distorting the meaning, it's a strong hint that it's a direct object. In this case, you can't say: "he went down (of/from) the stairs".

Il est descendu du bus. -- He got off the bus. "Le bus" is an indirect object, which is easily identified by the preposition (de). But even if in the English translation the preposition can be omitted, it can also be put in easily: he got off from the bus.

Hi Chris,

I imagine that you will already know that there are about ten of these so-called  "verbs of movement" that can take either avoir or être depending upon whether they are being used with a direct object (transitively) or without, via a preposition; (intransitively).

In the example, you have quoted 

this is transitive usage, therefore avoir, the direct object being "Le boulevard" He is traversing (or walking) down (adverbal usage modifying the verb traversing).

With respect to getting off a plane the sense is of "descending from" (preposition) therefore être is employed to express the "state" of descending from ....

Hope this helps.

Jim

"He went down the stairs" and "He walked down the boulevard" are both intransitive, because "down" is a preposition. On the other hand, "He descended the stairs" and "He descended the boulevard" are both transitive.

In my opinion, it's best to ignore the English translations, which might not match the transitivity of the French sentences. Instead you should just consider whether the verb in the French sentence is transitive or not.

Hi Alan,

In the two examples, you mention  --  "down" can also be an adverb - so in these cases, the verb is being modified by the adverb "down" and the direct objects are "the stairs" and "the boulevard".

The correct answer has been assessed as that of Chris above.