Is it halachicly allowed / appropriate for married men to be friends with other women?
And is it allowed for married men to drive every day with another women (married or single) to work?
It is definitely not appropriate or allowed for a man to be friends with other women. The fact that he is married does not in any way change any of the prohibitions regarding inappropriate relationships, etc.
Regarding driving with another woman everyday to work, it depends on the situation. If it is going to bring that they will talk to each other, which will eventually bring them close to each other, that is no good. However if they are just happen to be in the same car, and driving together isn’t going to foster a friendship it is allowed. Obviously this is only when their being in the car will not cause a yichud issue, such as traveling at night or a deserted road, etc.