Determine OSM highway exit information.
Turn-by-turn navigation apps use highway exit numbers and exit sign information to notify drivers when and where to turn off of major highways. OpenStreetMap has multiple schemes for tagging both the exit number and the exit sign data at highway exits. There is nothing stopping mappers from using multiple methods to convey the same information. It's possible that more than one tagging scheme is used on the same junction yielding different results. It's also possible that the exit is tagged in such a way that the underlying data is ambiguous as to which outgoing way is the actual exit, although a map might show the proper exit number in the correct location.
osmexit analyzes the tags at motorway junction nodes and their connected ways. It assigns the exit information to the outgoing ways in a uniform manner, regardless of which tagging method is used. Furthermore it reports when two different methods have conflicting results, and also when the tags are ambiguous and no assignment can be made.
Conflicting and ambiguous outputs should be cause for further investigation and cleanup using tools like to-fix or maproulette.
Note that there's the critical assumption that exit information can and should be determined by looking at the junction node and its immediately attached ways.
Basic Motorway Junction: Node tagged with highway=motorway_junction and ref=*, the exit number is the node's ref tag. If one outgoing way is a highway=motorway_link, then that way is assigned the exit number. If there is more than one motorway link, then the rightmost (leftmost when driving on the left side of the road) link is assigned the exit number.
Left/Right Junction: Node tagged with highway=motorway_junction and at least one of ref:left=* or ref:right=*. If there are two outgoing ways, then the leftmost way is assigned exit number ref:left and the rightmost way is assigned ref:right. If there are not two outgoing ways then this is an ambiguous situation.
Junction ref: Node tagged with highway=motorway_junction. Any outgoing way tagged with junction:ref=* is assigned that as its exit number.
No Ref: Node tagged with highway=motorway_junction and noref=yes. This exit and its outgoing ways have no exit number associated with them.
Basic Destination: Node tagged with highway=motorway_junction. Any outgoing way tagged with destination=* is assigned that as its destination information.
Basic Exit To: Node tagged with highway=motorway_junction and exit_to. If one outgoing way is tagged with highway=motorway_link, then that way is assigned the exit_to information as its destination. If there is more than one motorway link, then the rightmost (leftmost when driving on the left side of the road) link is assigned exit_to as its destination information.
Left/Right Exit To: Node tagged with highway=motorway_junction and at least one of exit_to:left=* or exit_to:right=*. If there are two outgoing ways, then the leftmost way is assigned exit_to:left as its destination information and the right most is assigned exit_to:right as its destination information. If there are not two outgoing ways then this is an ambiguous situation.