Agree, outside of the city would be more likely. However, how long would those traces remain? If it takes days or weeks for folks to start looking at roadways and potential spots, the traces may have been gone or ignored. I wonder what the weather was like in the days immediately after they were last seen? A moderate to heavy snowfall could have covered many signs of an accident.It is difficult to accidentally drive into a river, in Philadelphia, and not leave a trace on the bank, e.g. broken guardrails or fences, tire tracks.
Outside of the city, it would be possible.