Training Outdoors without power meter

I finally bought a speed and cadence sensor for my outdoor bike so I can in theory now do workouts outside. Not sure about how that would work though… I imagine I can download the workout into my cycling computer so it will tell me what to do when, right?
But what about traffic? How do I account for stops, having to slow down for traffic reasons etc?
Thanks for your input and your patience as this must be a very obvious thing for those of you who have been doing workouts outside all along.