I'm riding from Cumberland, MD to Richmond, VA. From Cumberland, I'm taking the C&O to Point of Rocks, MD, then crossing the Potomac and going south until I link up with the ACA Atlantic Coast route. Rt. 15 would be the best by car but perhaps a little busy for a bike.
DO NOT TAKE ROUTE 15. Odds are high that you will not survive.
There are much better/safer/saner routes.
From the C&O you can cross at White's Ferry and you'll only have a short distance into Leesburg on route 15, most of which has a rideable shoulder.
I've never biked Evergreen, but I've driven it and it seemed to be cautiously bikeable. Does tend to get heavy traffic.
Don't know about Watson Rd
On route 50 and route 15 near Aldie, be VERY careful. Road is narrow and traffic is heavy.
The other roads I don't know well, but Sudley can get heavy traffic.
If I think of an alternate route I'll post it.
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And here it is. http://www.gmap-pedometer.com/?r=3847215
Cross at Brunswick and take Berlin Pike. Traffic can be heavy, but a lot of us bike it with no problem.
Switch to the W&OD trail to Purcelville, or continue to route 7 business and take that west.
The roads from Purcelville to The Plains are good for biking.
Route 55 has a lot of local traffic, but most cars end up on I-66. Stay alert. I've biked 55 west of The Plains, but not east. Driven it many times and it seems to be bikeable.
November, Trek OCLV, Bianchi Castro Valley commuter
2nd cc's comment - Rt 15 from Point of Rocks south is unrideable without a very large truck behind you blocking all traffic. Better is cross at White's Ferry (+ snack bar & restrooms there), take Rt 15 briefly (there's a shoulder) then transition to Rt 15 Business into Leesburg.
Hand not thought of cc's suggestion but it looks like a good one. If you go further east towards Rt 28, you can probably work your way south via W&OD, Loudoun County Parkway to South Riding, may be some roads from there (avoid Rt 50 and 29, too). 28 itself has a side path that starts somewhere south of Centrevile, but north of that, 28 is a limited access high speed road where bikes are prohibited.
You might check the Potomac Pedalers website at bikepptc.org for routes thru Manassas.
....If you go further east towards Rt 28, you can probably work your way south via W&OD, Loudoun County Parkway to South Riding, may be some roads from there (avoid Rt 50 and 29, too). 28 itself has a side path that starts somewhere south of Centrevile, but north of that, 28 is a limited access high speed road where bikes are prohibited.....
That's a thought. But instead of Loudon County Parkway I'd take Clairbourne. Traffic is less crazy. Then just a little bit of LCP, Ryan Rd and Belmont Ridge. At Arcola, take Gum Springs to Sudley Rd. Ryan and Belmont have a lot of traffic, so be careful. I've ridden that way only as far as Old Braddock, so I can's vouch for the last part into Manassas.
Not as pretty as the roads in Western Loudon, but not as hilly either. It's mostly buildup suburbs until you get past Arcola. http://www.gmap-pedometer.com/?r=3856426