Uusimaa is a huge area, covering far more than Helsinki. I'm talking specifically about the metropolitan area.

I know nothing about commuting in Finland but is Uusimaa not roughly equivalent to "the economic gravity well of Helsinki"?

Sounds like you two are nit picking over the difference between the I495 loop being the "DC area" vs everything east of Fredricksburg/Front Royal/Fredreick being the "DC area".

I've no idea of the Geography you are referring to in the DC area, being European. That said Uusimaa is a region, Helsinki is a (capital city). Helsinki spans quite a large geographic area and is fairly densely populated but primarily in a metropolitan area. Take a look on google maps and you'll see it's tiny, say compared to London, Paris, Berlin etc. Uusimaa covers a much bigger area with huge amounts of space between the edge and the metropolitan area (mostly covered with fields, private land and forest).

Most of the population of Uusimaa (pop. 1.7M) lives in Greater Helsinki metropolitan area (pop 1.5M).

Yeah, but those 0.2M are spread out over almost as large an area as the 1.5; the Greater Helsinki area has a population density of 283 people per square kilometer, vs Uusimaa/Nyland without the Greater Helsinki area at 56. (according to https://sv.wikipedia.org/wiki/Nyland , https://sv.wikipedia.org/wiki/Helsingfors_ekonomiska_region and LibreOffice Calc.)

[EDIT:] So, sure, most of those cars must be owned by inhabitants of the Greater Helsinki area. Heck, I live in Helsinki proper, and I have a car. But... Up until last year, when I still worked at the office in the more central parts of town, my daily commute there was by bus and metro (and will possibly be again sometime soon). And AFAIk I was(/am/will be) far from alone in this combo. [/EDIT]