Wouldn't have arrested her, but she should have pulled over. I never seen, nor even heard of an impersonator using a fully marked car with activated lights AND sirens, pulling over someone. If she's so worried about her safety, she should probably stay within the speed limit, and not travel almost 20mph, above it.
This has happened several times in our area and we have been urged by the sheriff's department to exercise caution when pulling over. Here's a few articles for you to get caught up.
Man accused of impersonating police officer
Indiana Man Charged with Impersonating Officer | NBC Chicago
Man Suspected Of Impersonating Police Officer, Sexual Assault - Los Angeles News | FOX 11 LA KTTV