I have measured quite a lot with assistance from local police, with many of the jurisdictions in this area. They have been extremely helpful, and most have gone out of their way to encourage me to take whatever time I need to do my work properly and they would provide protection.
Most (but not all) police jurisdictions around here do require payment for these services, so I let the race sponsors work out the financial arrangements.
It's best to have a good conversation before you start, in order to explain what you're doing, especially how you might be riding erratically as they see it in order to follow the SPR. And even after that conversation, you'll need a good way to communicate what you're going to do (changing lanes, stopping, etc).
For some of the more challenging measurements I've found it helpful to have two police escorts; they can close down intersections just before I arrive, or close down two lanes at a time, etc.
For a number of years I've worked with the same 2 or 3 officers in the MPD (Metropolitan Police); a great advantage is they totally understand what I'm likely to do and how they can keep me safe.