Part 107 normally does not permit operation of an sUAS from a moving vehicle.
That said, if it’s a land or waterborne vehicle and you’re operating the sUAS over a sparsely populated area, that’s OK.
But how does the FAA define a ‘sparsely populated’ area?
The FAA has avoided defining ‘sparsely populated’ until they gain more experience and data with low-altitude flight operations.
You will have to use good judgment when considering an area to be sparsely populated.
The area where you’re wanting to fly needs to have very few people present, so if you needed to make an emergency landing, the impact on people would be slim to none.
If an incident/accident were to occur, the FAA would consider the area, the population present, and the manner in which the drone was being flown.
Prior to the Part 107 regulations going into effect to Part 107, one of the benchmarks used to define ‘sparsely populated’ was 20 people per 10 acres.
Breaking the numbers down to come up with a visual, you end up with 2 people per acre, or about 2 people in an area the size of a football field (actually 91 yards).
While this is not a legal definition and the judgment of the pilot is a key factor, it at least gives you a visual when thinking about a sparsely populated area.