Learn Finnish: Valitse turvallinen reitti.

Valita is a Type 5 verb. To make the casual command form (2nd person singular imperative), take the basic form, remove the final -a or -ä, and add -se. So, valita becomes valitse.

Why is the object turvallinen reitti in its basic form? Shouldn't an object have an -n ending?

Normally, yes! But commands are a major exception. When a verb is in the imperative (like valitse), its object drops the -n ending and looks exactly like the basic nominative form.

Does the adjective turvallinen also follow this special imperative rule?

Exactly. Adjectives always match the case of the noun they describe. Because the command form makes the noun reitti take the basic form, the adjective turvallinen also stays in its basic form.

What happens to the double tt in reitti if it isn't used in a command?

Reitti undergoes consonant gradation (from strong tt to weak t) when you add certain endings. If you were just stating a fact like "I choose a safe route" (Valitsen turvallisen reitin), the added -n ending would weaken the double tt to a single t.