How much risk does a heavy truck impose on highway safety? To answer this question, we look at the rapid influx of trucks during the shale gas boom in Pennsylvania. Using quasiexperimental variation in truck traffic, we isolate the effect of adding a truck to the road. We find an additional truck raises the risk of a truck accident - and, at an even higher rate, the risk of nontruck accidents. These accidents pose an external cost in cases in which the truck is not found liable, not fully insured, or not directly involved. We show this external cost is capitalized in the insurance market: car insurance premiums of other road users increase when trucks are added to the road.