If you're trying to game the detection system to minimize travel time then average speed cameras could encourage pathological behavior (which would hopefully also be caught by careless/reckless driving laws). They're dead-simple and not subject to cosine error and sensitive closed-source calibration techniques though. Even accounting for clock skew and clock drift, if an average speed camera says you were going x+2ϵ mph then it's safe to say you were going at least x+ϵ mph for at least some small interval and some very small value of ϵ.