"다리를 건너서 터널에 들어가세요."
Translation:After crossing the bridge, go in the tunnel.
-아서/어서 cannot be used in an imperative or propositive sentence. To connect such clauses, -(으)니까 should be used instead.
(2) But (으)세요 is not a polite request ending in this case.
(으)세요 actually is the combination of the honorific marker (으)시 (ref to 3rd party) with the "familiar polite ending" in 아/어/여요 (ref to the Listener).
Its application as a courtesy ending (not an honorific one) only expands to cover direct questions, propositions and commands in modern Korea.
So DLG example should read:
가다, to go
가-시-다 (honorific form of 가다, used with the honorific insert '시' )
=> (가-시-어요) 가시어요 (present declarative mode, familiar & polite style)
=> 가셔요 (contracted form) = 가세요 (Seoul pronunciation, officially recognized in c.1985)
다리를 건너서 터널에 들어가세요 = After crossing the bridge, "they" [Hon.3rd party] go in the tunnel. [Hon. declarative]
It's DLG Eng. translation that's a bit misleading.