“With the Atomic! perk, you are 25% faster and stronger whenever you're basking in the warm glow of radiation. Outside irradiated areas, your Action Points (AP) regenerate faster and faster the higher your level of radiation sickness becomes.”— in-game description
When eating foods that have a Rad effect longer than one second (such as Coyote meat), the perk will activate. Eating multiple coyote steaks will stack the Rads duration, giving the player a lengthy speed boost if many are consumed. The speed bonus functions even if over-encumbered, allowing a boost to quickly reach a vendor after fast traveling with Long Haul.
The speed boost applies to fast traveling as well, stacking the perk with Travel Light results in significantly reduced hunger, thirst and elapsed time when fast traveling under and over carry weight. For this reason eating irradiated foods with an effect longer than one second, or using player housing with continuous rads outside the door (Hell's Motel) can be beneficial.