marker can be illuminated under low light conditions and not when the environment is well illuminated the marker can also be configured to operate in either the continuous operation mode or the dark only mode depending on whether there is sufficient light to maintain the batteries in a good charge state in these designs a processor can monitor the voltage output of the solar cell via an internal analog to digital converter in order to distinguish dark conditions from light conditions and or can control the duration and or frequency of light flashing as a further alternative markers can be made with yet another operating mode in which light is produced only when particularly adverse visibility conditions are present e g when fog or other conditions result in substantial light reflection e g when roads are wet such that extensive fine spray is created by vehicle traffic thus the marker can also incorporate a reflected light sensor e g a fog sensor for example a phototransistor can be used to monitor the amount of led light that is reflected back to the unit this can also be read by another channel of the internal analog to digital converter in fog conditions or other types of wet conditions more light will be reflected back to the case when such conditions are detected the processor can pulse the leds the processor can also use another analog to digital channel to detect the state of the battery markers that include a reflected light sensor can be designed to operate in any of multiple modes for example based on input from a light condition sensor a fog sensor and a charge state sensor a device can be.