To the OPs original question…I’m not aware of it being illegal but I suppose an officer could press the issue and say vehicles are following too close. In all the years and miles of riding pacelines I (we) have been observed by a number of law enforcement folks and were never stopped. Certainly have known folks who have been stopped for running stop signs and lights. Off topic a bit, a pacelines can be challenging, fun and dangerous. I have made it a policy to break away (drop off the back) of any paceline who is joined by a rider or group of riders who do not typically ride with the paceline. One of the worst wrecks I was ever in was caused by a group of three riders which hooked up to our paceline, then when they moved to the front they overlapped and kissed tires. About 8 of us went down and 5 had to wait on a SAG.