As a native Russian speaker, I would say "по улице едет автобус" and "по морю плывет пароход", but at the same time, I would say, "этот автобус идет до вокзала", "пароход идет в порт" (in these examples the destination is mentioned an the manner of movement becomes irrelevant). I would also say, "Смотри! Мой автобус идет" for "Look! My bus is coming". It is common to say "поезд идет без остановок" (= it's an express train") and "поезд идет со всеми остановками" (it is a local train), but if you are on a bus/train, you'd say, "автобус/поезд едет быстро" for "the bus/train is going fast". When describing regular trips, we say "автобусы ходят по расписанию" (buses run according to the schedule) and "автобусы ходят регулярно" (there is a regular bus service), but "автобусы ездят быстро" for "buses run fast".
I got it wrong for saying "the blue bus drives along the street." I'll report it just to see, but I think it might be wrong because word order generally puts new information at the end of the sentence. Since blue bus is at the end, that implies we don't know which blue bus it is, therefore it's "a" bus and not "the" bus," but it would be good if a native speaker chimed in.
As the overwhelming majority of nouns ending in -а in the nominative singular and all of the nouns ending in - ица, улица is feminine. По requires putting the following noun in the dative case. After verbs скучать, тосковать and a few other verbs по can also be followed by a pronoun in the prepositional case (e.g. Она скучает/тоскует по вас = She misses you). A classical example of this usage is the Russian translation of “For whom the bell tolls” («По ком звонит колокол»).
The rationale is that the course is still in beta, so the volunteers haven't gotten all the possible translations programmed yet.
The rule of thumb I use is that the tends to be the preferred translation when the noun group is at the front of the sentence (known, definite information), and if the noun group appears at the end of the sentence (new, indefinite information) the preferable translation tends to use a/an. Take a look at the stickied thread about word order on the discussion forum if my rambling is not making sense. :)