Translation:Next, you go straight on the wide road.
I would think it's probably because 真 is read as ま (rather than say ます or まつ for example) and 直ぐ as すぐ so based on their readings alone you don't have any indication that it should be a long sibilant. I don't have any idea what they would have done before the development of hiragana, though.
Next, go straight at the wide street (marked as incorrect). Suggested correction: Next, go straight >up< the wide street... can someone help me see where I can read in the 'up' or 'along' with this sentence? I used 'at' because I thought >you see the wide street, and that's where you straight<
The use of を marks the wide road as a direct object. You're saying that your advancement is being done to the wide road. It's kind of a weird thing to say in English, but that's what is happening here.
If you used で instead of を then I'd probably take that to mean "at the wide road, [cross and] continue straight ahead." But I think if that was the intended meaning it would make more sense to actually say "cross the wide road." (Something like 次に、広い道を渡って、真っ直ぐ行きます。)
My thinking was the same! Especially since if you were to highlight the を in this sentence, it actually suggests the meaning to be "at". I was thinking along the lines of which way to go AT the wide road, i.e.: "take a left at the wide road".
The only thing I can come up with is: would you drive "at" a road, or would you drive "up/down/along" a road?