What are the halachos with taking a ride home, to work or a ride to the mountains with a male workmate?
Answer:
In general, it is permitted to take a ride with a male workmate, providing the route does not involve deserted areas where no other cars or pedestrians are present or likely to be present, and providing the the windows of the car can be seen through and do not have drawn curtains. At night, the roads travelled on must be well lit. Most in-town roads would thus be permitted to drive on at all times, but travelling on a small dead-end road at night would be prohibited. For journeys out of town the halachah would depend on the circumstances of the road.
Authorities mention as a general guidline of tzenius that one should seek to avoid "close" contact between sexes. It is better to avoid lengthy journeys, and poskim advise that a woman should sit behind, rather than next to a man.
Sources: See Minchas Shlomo (91:21); Devar Halachah (15:1, 15); Shevet Halevi (3:185), Mishnah Halachos (3:48). These poskim dispute the ruling of Iggros Moshe (vol. 4, 65:4) who is more stringent concerning yichud in a car.