This story was about traffic cops in the future that patrolled major highways in a RV-like vehicle that had food preparation facilities, sleeping berths for the off-duty officers and if I remember correctly an execution chamber for second offenders.
It sounds like this is a novella (then novel) called Code Three. The author was Rick Raphael.
"Get in Beulah, the massive 60 ft long freeway cruiser and patrol the law enforcement lane of tomorrows freeways with 2 male troopers and a female EMT in this Heinleinesque novella.
The freeways are a mile wide with lanes crossing over from 100mph to 400mph. The cruiser patrols the middle lane and in addition to being jet powered, had a an emergency surgery, food and sleeping quarters for the crew, a crane, and much more stuff including monitoring gear and a pair of .25 cal machine guns mounted up front. Hand out citations, clear wrecks, rescue people, fight the bad guys deliver a baby and chuckle at the repressed sexual tension between the crewmembers. "