If you haven't made the trip yet, here is some info for your questions about I-70 & I-270. Both roads are primarily patrolled by Maryland State Police (MSP).
If you're coming into the state on I-68 from WV, you'll first hit Garrett County. The McHenry Barrack patrols there, and they're on 39.08. You will next hit the Cumberland Bk. in Allegany Co., which uses 39.24.
Next will be the Hagerstown Bk in Washington County - they take over I-68 from Cumberland, and also I-70 coming in from Pennsylvania, so no matter which route you're coming into the state on, you will be in Washington Co. Hagerstown Bk uses 39.34, and is also patched to the County P25 system, but the patched TG is encrypted (their 39.34 lowband freq is in the clear).
The Frederick Bk. in Frederick Co. is next, and they use 39.40, and are also patched to the Frederick Co TRS, but the TG is not encrypted. Frederick picks up I-70 and also handles I-270 to the Montgomery Co. line.
Rockville takes over I-270 through Montgomery County, and also part of the D.C. beltway (I-495), from the Va. line to the Prince Georges Co. line. Rockville uses 39.32 and is also patched to the county TRS. Depending on your exact route into D.C., you could enter two other MSP barrack areas (College Park and Forestville). I'd be glad to post that info if needed. All of the above listed MSP lowband freqs use PL code of 110.9.
Safe travels, and be prepared for heavy traffic as you near the D.C. metro area, especially during rush hour.
