Using arcpy, update value in one field based on value in another

I am creating a level of service tool and I need to update the number of lanes field based on the operation type of the road. What I need it to do is: if the operation type equals 2W, then the number of lanes needs to be divided by 2.

for row in rows:
    for field in fields:
        if optype == "2W"
        arcpy.getValue(lanes / 2)