The signal range of a mobile tower can vary dramatically based on the radio frequency, radio power, antenna type, tower height, and technology type (2G/3G/4G). A GSM (900MHz) macro base station might have a radius of 20+ kilometres in rural areas. A 3G (2100MHz) base station might have a radius of little more than a kilometre or two in urban areas. It is very difficult to generalise. A pity that OpenCellID doesn’t have a way of capturing frequency.