It's always been a bit of a double standard.

If you are driving, the official answer is, traffic, no matter what the cause, "Get there when you can. Let us know when you have a best guess." As stated, communication is the key, be up front and honest as to what is going on. More than likely, if there is a major traffic event, other pilots will be affected as well as passengers. Now, don't make a habbit of being late, but it's actually a reasonable response from the company--shocking, I know!

The reason being, if you are being pressured to drive fast and have an accident, UAL becomes libel for a law suit.