light on too soon

Two streetlights on Rosslyn's Meade St. bridge were on at approximately 5:45pm today. Two other similar streetlights were not on.   Why waste electricity by turning on streetlights when there's plenty of light outside?

Update: a sensor on top of the streetlight turns the light on and off.  That allows the light's on time to track the changing sunset and sunrise.  Apparently the sensor is malfunctioning.  I've reported the problem to the Arlington County on Arlington's handy online streetlight problem form.