What goes into the algorithm Google Maps uses to calculate the suggested time of a given trip?

Does it assume the user will be driving the speed limit? Does it even have access to speed limits? Does it “know” where or how many traffic lights, stop signs, or school zones might be along a given route? It seems like for some areas the timing predictions are a lot more accurate than others. I know it’s just a guideline, but there are certain areas of town that are consistently wildly inaccurate.