Harvey,
usually the mass exodus starts about Labour day because everyone has to go home to get the kids in school and back to work. But there is still lots of nice weather left. Like Mbaston said, snow will not fly until the middle of Oct or later. 3 to 4 weeks back to Dawson Creek is easily done. Temps in August in the evening will be in the 50's, maybe high 40's here in Fairbanks. Remember, when we are cold and snow flying, most everyone else is OK because they are south of us. If your back in Dawson Creek by the end of October you will be safe, most of the time. If you are traveling up through Alberta, the 200 miles a day might be boring. The road to Edmonton is like going through Nebraska. Flat divided 4 lane. It's when you get about 4 or 5 hundred miles north of Edmonton that the scenery gets nice. If You travel through British Columbia, be careful of the Casiar(said cas e are. the e is long) portion. If it's been raining it can filled with pot holes that slows travel to a mere crawl and can do damage. But on the other side it is beautiful. When you get here 200 miles a day is doable. But sometimes the scenery doesn't change and there isn't really anything of interest to stop for, for maybe 4 or 5 hundred miles. (in my opinion of course, which I have lived here all my life, so most of this is old hat. ) The fishing in Valdez during September is awesome for silvers and halibut. Same with Homer and Seward. You can not throw you line in without catching something.
If there is anything else I can help with PM me or write on this thread. Hope this info helps.
WD