Lumens alone, of course, won't determine visibility or conspicuousness. Lumens is total visible light output and how that light is directed or focused has an equal if not greater effect. All high visibility taillights employ optics to direct light rearward in cones of various angular extent and with varying light distribution within those cones. The narrower the cone, the brighter the light will appear, and the observer needs need to be at the peak intensity within that cone for it to be the most conspicuous. These lights need to be aimed for best effect, and trade off off-axis conspicuity for on axis conspicuity.