Plain English...

Collateral damage for killing accidentally. Even unintentional deaths is a better plain speak than c.d.

I have never heard the phrase at all, but this is the definition online:

"A negative externality is a cost that is suffered by a third party as a result of an economic transaction. In a transaction, the producer and consumer are the first and second parties, and third parties include any individual, organisation, property owner, or resource that is indirectly affected."

It seems to have arisen in the economic sphere - I could not find any references to it in relation to RTAs, except as regards that economic spin-offs.

This morning DH was driven indoors by the strong wind so had an extra sneaky coffee and a look at Pawn Stars on ITV4.

I heard him laughing at an American specialist in Matchbox and Dinky toys describing a Coronation coach saying, 'There were only a few made the year the Queen was Coronated'

Lucky, I saw it in a tweet by Nikolas Christakis, master of Siliman House/College at Yale. I'll see if I can find it again.

Here it is. Oops! I spelt his name wrong; it's Nicholas.

American is a foreign language, so perhaps that explains the superfluous use of extraordinarily long words in incomprehensible explanations it.