De Beauvoir Cycle Gang Strikes Again

No, I'm not. I'm being precise. :)
You're wrong on that, Niall, and usage itself contradicts you. People say 'motor traffic' or 'pedestrian traffic' or 'cycle traffic' all the time. Just Google the phrases and you'll get thousands of hits--1,730,000 today for 'pedestrian traffic'. Of course I know that people mostly use the lazy shorthand, but this serves to obscure the actual meaning of 'traffic'. Collins definition: 'the movement of vehicles, people, etc. in a partciular place or for a particular purpose'.

People do not say it all the time. People in a particular field of interest may write it in dry technical documents and web pages because they wish to be precise.

It's not lazy shorthand, it's perfectly valid colloquial usage. For it to be lazy the person using the term would have to be aware that they should be prepending the adjective, and for the most part they are not. Is "United Kingdom" lazy shorthand for "The United Kingdom of Great Britain and Northern Ireland, its territories and dominions?"

To use the term 'motor traffic' yourself may be considered precise, but admonishing someone else for not using it is definitely pedantic.

It is of course possible to shape language deliberately rather than to leave it to pure accident.

That's the sort of thing only a German would do :-)