Need help with masking a time-lapse animation

Question

Hi all,
I'm working on a time-lapse project where I have taken 240 shots of a highway heading into my city, and have erased the cars from the shot, creating the look of an "empty city". This will be one of many sequences like this.
I got the highway background plate perfect, no cars, and I threw that into my AE comp (CS6), along with the 240 images as a JPEG sequence. I've layered the background plate on top of the image sequence and used a mask to "hide" the cars from the highway.
The problem is, some of the cars are taller than the roadway (trucks, busses, etc), so they push into the upper part of the frame where the motion is supposed to happen. If I extend the mask upward, you can see a literal bisection between the still plate and the animated JPEGs. If I just move the mask path to hide cars as they go by, the moving mask becomes VERY obvious in the exported QuickTime.
I've attached a still of my setup and a short video for explanation. If anyone can help me salvage this shot, I'd be very grateful. I do NOT want to have to erase out the tops of the cars in every photo I took

Answer 1

Dave LaRonde wrote:
A lot of planning went into those shots. 
You'll note that in many of them, the things that move -- clouds in the sky, for example -- can easily be masked, and they were shot time-lapse over a short duration of time.  In other shots, the sun wasn't an issue: it was cloudy or it was night.  Others were assembled shooting in real-time with masking, and again, changing light angles would not have been an issue.  
There are a lot of different techniques used in those things.  It's interesting to note that at this web site, the photographer/editor/VFX practitioner only mentions some of them: particularly the use of clamps instead of tripods.  He doesn't give away many of his tricks.
I presume you've doped out how the zooms, pans and tilts were done.
Easily masked is I think where I got tripped up. In the Golden Gate shot, for example, it's pretty much a straigh mask up the center of the shot. In mine, there are curves with lots of buildings right next to the highway. Poor choice of location, perhaps.
Yes, Ross definitely doesn't share a lot of how-to, mainly just erasing objects and layering in After Effects. How he deals with problems/challenges isn't covered, so that's my learning curve.
The zooms are easy, you can do that in post if you shoot high enough resolution. The pans were done on a pocket dolly using a remote device that automatically tracked the camera across the scene over time. Couple hundred bucks to rent one, but too advanced for me at this point. I'm more concerned with getting stable, usable shots.