Baltimore is a very distinct city with a distinct identity, although the border between DC suburbs and Baltimore suburbs is kind of vague; I wouldn't consider Baltimore part of the DC area.

My general cut of it would be Frederick - Leesburg - (follow US 15 south) - Gainsville - Quantico - La Plata - Waldorf - Bowie - Laurel - back to Frederick, although I'm not high confidence of the cuts on the MD side of the line.

It does look like that maybe cuts MD tighter than VA: https://app.traveltime.com/search/0-lng=-77.03656&0-tt=45&0-...

As my sibling comment points out, VA sprawled a lot further than MD did. The US-15/Quantico line in the VA is really quite close to the boundary between suburban sprawl and true rural. Cross the Potomac, and you cross from sprawl on the VA side to rural lands on the MD side: the western and northern reaches of Montgomery County are definitely rural, similarly for the southern reaches of Prince George's County.

An additional factor to consider in the DC area is that the DC central business district is relatively weak compared to other major jobs centers: Arlington, VA (just across the river) has hefty job concentration, as does the Dulles-Tysons corridor; on the MD side, there's an additional jobs concentration on Rockville-Bethesda.

The final factor is of course the Baltimore-Washington divide. As you head northwest in MD, more people start commuting to Baltimore instead of Washington. So instead of there being a relatively clean sprawl/rural divide you can point to as a boundary, there is instead a more or less continuous sprawl that transitions from DC suburbs to Baltimore suburbs, and the mixing zone (particularly the Laurel-Columbia belt) is more accurately a suburb of both rather than one or the other.

Virginia wanted to grow its exurbs, and Maryland didn't. Virginia created a lot of large houses on former farmland, where Maryland preserved more of it.

Maryland also did a better job of spreading out its employers. A lot of those Virginia exurbs still commute into DC, or at least Northern Virginia, making traffic a nightmare, at least during rush hour.

Another thing that slightly confuses that map: Virginia has much better arteries into DC. You get into DC from the south on I-395 and I-66, and they take you all the way downtown. Maryland has only surface streets. (It was supposed to have I-95 connecting straight through the city to join up with I-395, and I-595 where New York Avenue is, but that would have destroyed a lot of neighborhoods in exactly the way they were destroyed in building 66 and 395.)

That means that there's a fair bit of Virginia that is technically 45 minutes away from the center of the city, but not during rush hour. The 45 minute line in Maryland is pretty close in, but the 1 hour line turns out to be quite broad, because you can reach it on Maryland's interstates that flow pretty freely (parts of it, even during rush hour).

Of course you really should be taking public transport, except during a pandemic. The driving and parking are both horrible.