Even before they could cross St Lorenzo Avenue, Luke already could already see tons of black smoke coming from the direction where the airport was supposed to be. The aqueducts of fuel probably had been damaged and exploded. That would explain so much smoke. In the end, using the airport was really out of the question.
Amanda didn't think that Luke would work with them again after what happened. Still, he surprised her since he was being reasonable for a change, but she knew that it was just something temporary due to the situation. Maybe Luke could escape the city by himself, but he couldn't do it in less than five hours.
Since they couldn't use the airport, then Luke assumed they had to head west, which was the direction the chopper came from when that pilot attacked him. The shortest route out of the city was 8th street, so Luke gave the order for Amanda's father to head toward there.