"If you are cleared to back taxi, are you cleared to cross runways during the back taxi?" - Howard M.
“No (you are not cleared to cross runways during the back taxi). The Aeronautical Information Manual states the following:
A clearance must be obtained prior to crossing any runway. Air Traffic Control will issue an explicit clearance for all runway crossings.
What this means is that you must be careful not to cross a runway while taxiing unless you are told you can cross that runway. If your taxi route requires that you cross multiple runways, you will have to deal with each runway crossing individually.
This is why an airport diagram is so important. Without an airport diagram, it is difficult to be sure that the taxiway you are about to cross is not a runway; or if it is a runway, that it is the one you are cleared to cross.
The situation becomes more difficult at night.
Taxi slowly and stop if you need to refer to your taxi diagram. If in doubt, STOP and ask ATC for clarification of the route or ask them for progressive taxi instructions. They will be happy to help. This applies to both day and night.”