Bus between Mendoza and Santiago: are there precipitous drop-offs on the edge of the road?

This is one of my favourite bus trips in the world, having done Mendoza->Santiago twice and Santiago->Mendoza once. Please, please, travel it during the day, for two reasons:

1) I've done this once at night. At the top of the Andes is the border crossing. There's snow a good portion of the year on the hills around you. It's FREEZING in the middle of the night standing in this big warehouse waiting for your turn to get a stamp.

(from marksmayo.com)

2) It's spectacular. On the way up you can actually see the Inca Bridge from the bus - keep a look out for road signs. Just before the Argentinian border, if you know where to look on the right side of the bus, you have a 20-30 second window where you can see Aconcagua. The mountains are fantastic, and the roads are actually really good.

Now, in terms of drop offs. The final time I did this trip was with a friend who does NOT have a good head for heights. Heights freak him out something chronic. He was fine the entire bus ride. The roads are good, they have tunnels at key points rather than make a risky road or risk avalanche.

Checking my blog, I didn't provide much description, but...

• The first two crossings - a night crossing and a day, are mentioned in this post
• The final crossing back to Santiago is in this one

Now, there is ONE key point that might freak you out a little bit. I actually mention it in the first post above, as:

Just before that is a massive series of switch-backs going up nearly vertically, part of which travels UNDER a ski-lift!

This is through the pass called Paso Internacional Los Libertadores and is a series of 27 switchbacks on the Chilean side. It becomes almost monotonous going up, but the view going towards Chile (down) is spectacular. But even my friend with a fear for heights marvelled at it (and he did the trip twice).

(from Wikipedia)

But the road has lots of traffic, the buses are very good quality, and the trip takes about 8 hours. It's a fantastic way to spend a day.

(EDIT) If you really want to see what it looks like, there's a video of some of the switchbacks showing the drops, but also the inside of the bus and how slowly the bus goes to make sure it's safe. You'll be fine :)

The photos and descriptions on this website might give an idea of how it's like:
Crossing the Andes: Mendoza to Santiago by Bus