@Karen Davis good question, and honestly for your specific use case I'd say you're probably fine — 30-40 km/h is slow enough that Doppler shift on a 2.4GHz BLE signal is essentially negligible. The frequency deviation at that speed is on the order of a few hundred Hz against a 2.4GHz carrier, which the receiver handles without breaking a sweat.
The thing that'll actually matter more for you is antenna orientation on the vehicle-mounted node. If the antenna is omnidirectional (which it should be for mobile tracking), you won't see meaningful pattern nulls just from the vehicle moving in a straight line. Where you'd start noticing issues is if the antenna gets body-blocked by the vehicle chassis itself — like if it's mounted low and the vehicle body is sitting between it and the base station at certain angles.
For a rooftop base to slow vehicle setup, I'd focus more on keeping the vehicle node antenna clear of metal interference (mount it high on the vehicle, away from the roof panel if possible) than worrying about motion-induced degradation. The 50km figure is drone-to-drone with no ground reflections, so ground-level vehicle to rooftop you're realistically looking at the 5km ground-to-ground envelope, but within that you should see very consistent performance regardless of whether the vehicle is moving or stationary.
