I'm not familiar at all with Police/HEMS but I would guess
1. Busy town in Oxfordshire
2. Major crossroads in said town
3. Narrow roads and hence small space in which to land
4. Potential presence of bunting across street
5. Traffic lights
6. Locals, feckwits and ijits
7. Bollards, road humps and sleeping policemen
Cheers
Whirls