Trek recommends lights on at all times
Cycling has taken some of its best cues from the automotive industry. These days, cars come standard with daytime running lights because cars with DRLs are less likely to be involved in a collision. Applying that logic to bikes is a no-brainer. Daytime running lights are shown to significantly reduce the likelihood of collision. If you take only one safety measure to increase your detectability on the road, equip your bike with daytime running lights and use them on every ride, day and night.