I have always found google maps to be a great assistance in finding the correct route in my local area. If you can see all the roads that could lead to your destination you can then pick the best route for your self.
Good advice.
I've commuted a 10-mile round trip to work for about six years now. I started pedalling a cruiser bike, then electrified it. Top speed was 27 mph, which kept me on the bike path, sidewalks and city streets. Next was a gassified Dahon folder, which still required me to follow the same route. The cruiser was then fitted with one, then two Mitsubishi 2.2hp engines. This setup's top speed was 39 mph, which finally freed me from the bike path. Nimitz highway had a long six-lane highway, but speeds over 40 mph make for a hairy ride. Outfitting my MB's with 4.2 hp GP460 and 2.8 hp Tanaka engines allowed me to ride faster than 40 mph. Moreover, it helped me find the safest routes where I could keep up with traffic travelling less than 40 mph. Even though there are moderate hills involved, the lower speeds and less volume of traffic makes the safest route the best way to commute to work. The last leg of the trip is a long stretch with 25 mph speed limit, so that's safe and less stressful for bike and rider. There is a cop who strictly enforces the speed limit there, so that's nice.
I think the safest route depends if you can ride with cagers at a respectable pace. It also depends if you take the lane or hug the curb.