You're right about the zone for busses. The North West corner (top left) could be an area for buses and taxis with a "drop off" zone for public use. The North East corner (top right) could then be the staff car park, or reserved parking for executives. Even my local station has these things, so you're right that they need to be included.
Here's my updated proposal:1024 x 768, 216kB
A design showing what you've actually got in mind might help to clear up the things which don't seem right or aren't included. It doesn't have to be neat but it does need to show the things you are suggesting.
If the road only descends 15m then it will only be at sea level. The tunnel to Port Orchid needs to be on the sea bed; tens of metres below the surface. Your freeway would need to double back on itself at least once get down there.
A gradient of 1:4 is too steep, especially for a freeway. Heavy vehicles (such as articulated lorries) cannot climb these at more than a crawl. Your freeway would need a shallower slope, with more turns. By using the circumference of a long oval we can minimise both the gradient and the sharpness of the turns required to reach the sea bed.
Where will the ramps and underground parking in your design be? How will vehicles move from one level of parking to another? With the oval, the underground parking fits neatly into the middle of the freeway spiral. Vehicles would use the spiral to go up and down the levels, just like a conventional multistorey car park.
American cities and airports are very large so these underground systems are less rare. GTASA's airports in Los Santos and San Fierro mimmick the underground systems present in real airports. I've travelled through curved motorway tunnels even here in the UK. In Germany, Italy and other places with a lot of height changes it's common for tunnels to be curved. Driving around the underground stuff in GTASA is actually quite fun.
The radius of the kink at the Southernmost point of the route is a tiny fraction of the illSUB curve it leads to. There is a length of straight track between the kink and the curve which could be used to help smooth the kink. From the aerial shot you gave, my proposed smoothing didn't seem to go through any buildings.
(EDIT) I've just been showed a set of new renders by illspirit to figure out the details of running the train up the East coast of illURB.
The 3D draft proposed that the bridge from illCOM should slowly begin sloping upwards about 200 metres before it reached illURB:illurb-east, 1053 x 414, 48kB
After much debating and drafting, we've settled on a combination of changes. The first hbump is extended so that a small railway station can go on it. It will also give the bridge between the bumps a proper foundation. The bridge will curve slightly West and may be slightly uphill. When it reaches the second bump, it will immediatly enter a tunnel in the cliff face.
We played with various routes, settling on a smooth curve. It is shown in yellow with dark yellow indicating a tunnel:illurb-east-smooth, 746 x 767, 43kB
But the new land will look like this:bbqtrain5, 1024 x 768, 283kB