Where I live, I've seen the police park their cruiser at an angle behind the vehicle they are stopping. The problem is that it renders their LED light bar almost invisible. I guess they don't realize this.
I'm wondering if certain brands/ models of bars have this problem more than others. I do know that some LED's have wider viewing angles than others. I think in the future, SMD LED's will be the norm, and this will be much less of a problem.
I've never experienced this myself, but I think I like the old rotating lights rather than the new LED lights on emergency vehicles (think the NYPD patrol cars). Anytime I've seen a police car with it's lightbar active that has pulled someone over, they have been clearly visible, and that's both the NJ State Police or just the local municipality cars.