In these cases the interviewer should understand and if possible, reschedule your interview.
When you are late, however, because you did not plan properly, then that is just rude and disrespectful.
The following are a few tips to ensure you arrive on time.
Make A Practice Run.
If you are travelling to a place you are unfamiliar with (assuming it is within driving distance) make the trip beforehand to see how long it takes.
This practice trip should be made near the same time of the day as your interview so that you will be able to judge such factors as traffic.
If you are traveling from out of town and arriving the night before, you should become familiar with how to find the location the night before the interview.
It may be worth your while to take a taxi ride past the building to get a better sense of the distance away and the time it takes to get there.
Ask the driver how long such a trip would normally take around the time of the interview.
While you should arrive at the interview itself 15 minutes early, arrive near the interview location 30-60 minutes beforehand.
Go to a coffee shop and relax for a few minutes.
You can read a paper or go over your resume.
After you have relaxed and mentally prepared yourself for the interview, head for the interview itself, ensuring that you arrive 15 minutes early.
Park In A Parking Garage.
You have arrived early, don't mess it up by circling the block a dozen times to find cheap or free parking.
The few dollars you spend in order to park will be a small investment in helping you get the job.
Okay, in spite of all this you still find yourself running late.
If you find yourself in this situation make sure you call the interviewer.
The interviews may be running late, which means they might still be able to interview you.
If you will not be able to make the interview, it is only polite to let them know so they don't end up wasting their time.
It may also allow you to reschedule another interview.