I've gotten questions lately about whether all police officers and detectives have partners. Actually, most patrol officers and detectives do not have partners despite what you see on TV. A few very large cities are able to partner some of their officers but most law enforcement agencies simply can not afford to do so. It's terribly expensive to add an additional officer to a patrol car, not to mention a very inefficient usage of available staffing.
Research shows that a two-officer car is not much safer than one-officer cars. The thought is that an officer alone realizes he or she has to watch out for danger and operates at a heightened state of vigilance. With two officers, it's possible each thinks the other is watching out and thus drops his or her guard. I don't know if that's true but as a former police chief, I know most agencies can not afford to staff two officers to a car or pair up detectives. It is not economical and it's not as effective. Two detectives following one another around aren't going to accomplish as much as each working his or her individual cases. Two patrol officers in two cars can cover more ground and deter and detect more criminal activity.
That doesn't mean that detectives don't often pair up to go out into the field looking for suspects or witnesses. But they usually aren't "partners" like you see on TV and in movies (Lethal Weapon, Starsky & Hutch, Cagney & Lacey) where detectives almost always have a constant companion. Most detective work takes place behind a desk. When detectives need to go out and need backup, they grab whoever is available to be "partner for the day."