I would like to know traffic statistic information in order to for example predict how long tomorrow I will spend to go from home to work in case I will get out one hour later than usual. I suppose Google should have such information, but how I can retrieve it?
Go to Google Maps and get driving directions. (For instance, Downtown Washington DC to Dulles Airport.)
On the left, under the start and end points, is a menu that says "Leave now". Click that and change it to "Depart At". Modify the time and date to your projected departure time.
Your projected routes will be adjusted for what traffic typically looks like for the day of the week and time you're looking at. (Here's DC to Dulles at 5:00 PM on a Friday.)
Note that you can't change your depart/arrival times if you use the multiple destinations feature.