The speed limit!
45 MPH from La Canada to Mt Wilson, 55 MPH after that to Nukes.
Too much traffic and cops to speed up there, just isn't wise.
I spend a lot of time up there, there are some great mtn biking and hiking trails/fire roads off the road from ACH up to Mt Wilson. There's a nice fire road hike/mtn bike ride to an old tram/rail that's pretty cool. Plus, many more.