When I was younger, I consistently ran 30 miles a week. But I’m tapering off with age. Last week I only ran 10.

You use the expression "taper off" to describe something that diminishes slowly over time. An example sentence is: "during the fall, tourism in London tapers off". Another is "you should taper off your caffeine intake".

Traffic does taper off, so do some other things, like pressure, problems, etc. The mast on my yacht tapers off in the higher sections. To taper, means to slowly reduce, or get thinner. Not common, but a normal word.