I have to disagree with this suggestion. The hydra is supposed to be a fast way to transport yourself, no matter what's your occupation. I would recommend that instead of this suggestion, why not increase the freezing time if you've left the hydra as a cop and a wanted player is nearby? This is better than not letting them leave the vehicle, it's not fair, to be honest. Voting negative.
I understand your point but making it disabled from a specific KPH isn't a good idea, disable leaving Hydra's at a specific altitude would be more better, this will disallow anyone to abuse wanted criminals with their hydra or block any kind of entrances.Positive (If it's for the altitude thing not the KPH)