The environmental argument for speed limits is ridiculous because many places have a carbon tax/other taxes meant to capture the unpriced environmental cost that accrues to the commons. With the carbon tax the person doing the speeding is paying for any excess harm they to the environment they may be doing and is willing to pay that cost in order to go faster so they should be able to go faster. If you don't think the cost of carbon is properly priced argue to raise your carbon tax, don't argue to add additional special taxes for various things you don't like.