If you really would only feel safe riding on the dirt shoulder, I'd keep that route as a last resort, since dirt will probably slow you down more than traffic lights and 55 mph traffic is likely scary and dangerous. On the other hand, lights might be more annoying than an overall slowdown from dirt.
Try one, then try the other. Time yourself. Weigh how pleasant the ride seemed. Try each of them each way. Pick the one that was faster, or the one that was more pleasant, whichever matters more to you. It's likely the faster one will also be the one that felt more pleasant, since losing momentum is a major annoyance for most of us cyclists.
Personally, I find stop signs slow me down more than traffic lights, especially on a route that I take frequently. With a traffic light you can learn the timing and either stop pedaling or pedal harder in order to avoid hitting the red.
I can tell you how I chose my routes:
- Look up routes on maps. Google maps has some decent cycle route data now. Turn on the "bicycle" and "traffic" overlays. My local bicyle coalition also has a map showing which roads local cyclists prefer (and has better data on bike paths). It's likely your local government has an official map of where bike paths/lanes/routes are. Get multiple routes figured out. Look for side roads, bike paths, etc. Small residential roads you'd never consider in a car often work great on a bicycle. And google maps may not suggest that awesome side road because it's not designated as a bike route.
- Pick the top few routes.
- Ride the best seeming one. You can do this as a test ride on a non-work day, especially if you're concerned about timing. Keep your eyes out for possible side roads, bike paths, etc.
- Ride a different one. Repeat until you've tried all the reasonable seeming options.
- Try them in the reverse direction. Sometimes what you don't like on a route won't be there going the other way.
- Take what you've learned and adjust.
I have two basic commutes, a mixed-mode with 4 miles of bicycle ride, or a 12-mile ride all the way to or from work. In both cases, my favorite route to work is very different from my favorite ride back home, except for the half mile closest to my house and a few miles in the middle of the 12-mile trip.
Roughly ordered priority of selection criteria for picking routes, based on thinking about why I take the routes I do:
- One bad spot can ruin a route. All perfect smooth bike lane except for that intersection that you feel like you're going to die in half the time? Terrible. Try a route that avoids the bad spot.
- Avoid bad road positions. For instance, a busy narrow 55mph road that's one lane in each direction with no shoulder would be bad. But if it's plenty wide with a wide shoulder that would be fine. Or a narrow low-traffic 25mph street would be fine.
- Avoid difficult turns. Needing to merge across lanes of busy fast traffic is what I find the worst. There's other ways to make a left turn, but they're usually slower.
- Avoid long waits. Signals that don't pick me up (or just have a very long cycle), stop signs facing busy road that doesn't have to stop.
- Avoid annoying traffic patterns. Signals or stop signs that always have a long line. Places where everybody makes a right turn and I want to go straight but it's hard to merge into traffic, etc.
- Avoid difficult hills. I don't return home on one road that I love for getting to work because it ends with a short steep hill with a left turn. I prefer long gradual slopes over short steep ones, but you might find you have the opposite preference.
- Find bike lanes. Or a good shoulder with not too many right turns across your path. I believe most of the US classifies these into 3 classes: class I is dedicated bike-and-pedestrian-only ("bike path"), class II is a bike lane, class III is a route/suggestion (but might have sharrows or other measures). I usually prefer class II, since class I usually involves dealing with pedestrians and most of the class III stuff around here is a joke with better side-street options.
- Find smooth pavement. Avoid potholes, badly maintained road, etc. Hopefully obvious why. I find sometimes the pavement going one way is much better than going the other.