Be aware that the Beltway (I-495) on the Virginia side of DC and I-95 south of DC has introduced EZ-Pass on their HOV lanes.... if you have more than 3 people in your car, you still need an "EZ-Pass Express" to access the HOV lanes. If you don't have 3 people, you can still access with an EZ-Pass, you just pay more.
A little confusing on exactly which type of EZ-pass you need... I think there is a special one with a switch you set which indicates you have 3 people (or more) in your car. In addition, the Beltway express lanes on the Virginia side (I-495) have varying toll rates depending on how congested traffic is. If there is a lot of traffic, the rates go up..... not so much traffic, the rate goes up. Plenty of signs display the current rates prior to the exits onto the Express lanes. I'm not sure if this varying rate applies to I-95 HOV south of DC.
Only DC could make driving this complicated !
More info at:
https://www.expresslanes.com/