That "road" is in fact a taxilane, and marked as such on the Paine Field side (note the yellow lead lines marked at the bottom of the image below, you would find on a taxiway). It connects the Boeing plant to Paine Field's "Boeing Ramp" where final prep can be completed before the aircraft are flown off to wherever they're going.
The terminology is important here: The bridge acts as a taxilane (a non-movement area not under the control of ATC which connects various aircraft parking areas to the taxiways). Boeing coordinates their own movements between the main plant (north) and Paine Field's Boeing Ramp (south). (The bridge is also not depicted on the airport diagram and may not even have "taxilane" status - as far as ATC is concerned. It's just sort of "there".)
I retired from Boeing and worked at the Everett plant for the majority of my career. We always moved the aircraft from the factory apron across the bridge to the paint hangars with a tug.
The aircraft are not fueled until ready for engine and other ground pre-flight tests. The engines are never fired up unless there are intake cages and blast fences in place. There are also FOD sweeps made to insure no foreign material gets pulled into the engine intakes. Furthermore, the “jet blast” would be destructive to property and the personal vehicles parked on the factory apron. In addition, it would require a certified individual to “drive” the aircraft under power.
The bridge is the connecting taxiway from the main factory to the paint and finish facilities (the three large hangars on the right side of the photo), flight test and the customer delivery centers. ‘Green’ airplanes are moved directly from final assembly to this area via the bridge. This usually takes place in the early hours of the morning to reduce auto accidents caused by distracted motorists on the highway below. It is quite a sight; a 747-400 crossing that bridge looks something akin to an elephant trying to walk a tightrope.