The fastest route for me has always been from Lee downtown take Thompson Mill Road, cross 125, then Mill Pond R, turning to McCrillis to 152, then Halls Way (Nottingham cut-off) then S on 156 several miles to Mt Road turnoff that gets you to the Park entrance. It is likely that this is what you got from Siri. There is no other route that is faster.
From 156, Siri had me taking Deerfield Rd to Round Pond Rd. which I didn't like the looks of & didn't try. And my "amateur" continuation (ie, blind faith for once) with Siri's subsequent suggestion...I'll tell you how that unfolded sometime elsewhere. Of course, be sure to include "park headquarters" in one's request for directions.
"Don't take your eyes off the road or it may lead you elsewhere".
Yes, that is the washed out road which has not been in existence for probably over 30 years. It is now the hiker's access to the boulder field (climbing cliffs) and the trail to N. mountain.
I saw the satellite view of it last night. Something to explore in warm months. I have not looked at non-O parts of Pawtuckaway in years. I don't believe I have ever been on Round Pond Rd.