Can you report a crash landing on an emergency frequency even if you are not at the airport?
If you are in a car, you probably don't have the full picture. Chances are, if the accident happened "near the airport", emergency services are probably already on the way. This is because the pilot would have been communicating with ATC, and would probably have indicated that he was in trouble before the actual accident. Even if the pilot did not have time to notify ATC, if a plane suddenly "dissapears", search and rescus will be initiated.

In any case, if will do no harm to call the normal emergency services (dial 112 or 911 in some countries). Don't overcomplicate things and try to get in touch with ATC - just dial the emergency number directly, and they will know what to do.