Since lumens is a measurement of the total amount of light being emitted, yes, the output of multiple leds adds. As CelticCross74 was stating, a more relevant measurement is candelas, which is a measurement of how much light is falling on a given object. The more candelas, the brighter the reflected light, and logically, the further away it can be seen. Candelas are mainly a function of the reflector/lens setup used by the light. As an example, yesterday I compared the beam from my trusty TM11 and my new TM16, at roughly the same lumen output of 2000 lumens. The four reflectors used in the TM16 produce a much tighter beam than the three used in the TM11. The result, a smaller, brighter hot spot in the center of the beam. That is one of the reasons we are always calling for comparison beam shots when a new light appears.