When a maps app/site or GPS gives an estimated time of arrival for a destination, in what way do they calculate for traffic lights?

A friend of mine and I were discussing today a possible dinner destination. I suggested a restaurant that, when I checked on Google Maps, is the exact same distance, time-wise, from my house and her house. It shows 38 minutes for me to get home and for her to get home, even though it’s 10 miles longer for me (probably due to highway and her’s is local roads). When I emailed her about the location suggestion and told her about the 38 minutes for each of us, she stated that her time will be longer due to traffic lights (which I took to be she’s making a negative out of what I considered a positive).

I am just curious, how do map programs and GPS calculate for time for traffic lights? Do they go on the assumption that all lights will be green, all lights will be red, or 50/50, or something else?