The presence of a heavy truck on the road can impose an externality if accidents occur that would not have otherwise. We find each additional truck on the road increases the risk of a truck accident—but also, at an even higher rate, the risk of a car-on-car collision. Our estimates imply two percent of all car-on-car collisions can be attributed to trucks on the road. This negative externality falls on all road users through higher car insurance premiums: one truck, driving for a year in the same zip code, increases the insurance premium of each new enrollee by $0.48/year.