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...

This paper assumes that workers can move from a market with high unemployment to one with low unemployment at a cost. In principle. equilibrium mobility can be greater or less than the social optimum. For most plausible parameter values. however. mobility is too low. Intuitively. mobility has a...

Climate change is a global "free rider" problem because significant abatement of greenhouse gases is an expensive public good requiring international cooperation to apportion compliance among states. But it is also a global "free driver" problem because geoengineering the stratosphere with...