Power: If you charge 6 hours per night a standard 18650 cell should do the job. You could use a TTGO T-Beam with integrated battery holder/charger for this.
You may check the power of the traffic sign. If the lightning is LED there could be some kind of power supply you may use. Otherwise use 5V USB charger.
Activation: if the goal is to save power this could best be done by using deep sleep mode, but this is not programmed yet. Most power consumption comes from Wifi stack. Take into account that if you load the Wifi stack in the moment when a pedestrian arrived, this could be too late, since starting the stack takes some seconds.
Also take in account that you probably also will count the pax in the waiting cars.