Random details for travelers: if you're going westbound, and if it's open, the left-hand exit onto Monument Dr will save you lots of time getting to Fairfax Corner. It's HOV-only during rush hours, and it's only open in one direction at a time. There are signs before it, although you don't get much notice before you have to make a decision (Monument vs 50 West vs 50 East).
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And if you're traveling eastbound, I don't think there's any way to know whether the ramp back onto I-66 East is open until you're on Monument Dr approaching the I-66 overpass:
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