Going out to lunch a few times a week with a group of co-workers or even a single co-worker who is the opposite sex is one thing.
Going out to lunch everyday with the SAME opposite sex co-worker is a different story in my book. If that is the same co-worker you are spending an additional 3 hours a day alone in the car with, you are then spending at least 4 hours a day with that person with next to nothing interferring with your time. I think it's totally natural that it's possible feelings could develop. Great conversation, no stress, no pressure to feed and bathe the kids, especially for a male who has a woman that is talking WITH him and not AT him to get things done in the house.
To me it's similar to letting teenagers have co-ed sleep overs etc. Sure if they want to have sex they will regardless. Why make it easier? Why set up the breeding ground for something to happen? Marriages have ups and down as a PP said. A great week with the traveling co-worker and a hard week at home could be the beginning of a disaster no one saw coming.
Even for the greatest guy in the world.
I just asked DH what his answer to the OP would be & he said it would depend on the guy.
) , but he is very much traditional and likes to do things how most people do. He had a single mom growing up, so as far as marriage, he doesn't know anything about how it can work. He says, if this is normal and most people would do this, then for me to go ahead.

If you fall in love it will be to late or painful.
There are actually more, but I will stop here. All of these started as "friends or colleagues." I know that not everyone will take the path these took, but why take a chance! Build up the relationship with your spouse. Don't do anything to put it at risk.