Automated Driving

You may want to consider getting this mod instead. I haven't tested it, but it seems like it would be better. Choose your desired cruising speed and take your foot off the gas pedal.
J - Turn on cruise control
L - Turn off cruise control
I - Increase speed by 1 unit (roughly 2mph)
K - Decrease speed

There is nothing in this stopping you from abusing it and rocketing through the air at 400mph through the air in a boat. But if you use it as intended, you'll cruise along at a comfortable speed.
(There is an upper limit on speed which I implemented to keep your game from crashing, though you may remove it if you wish)

This is useful if you like driving slower than max speed, as you don't have to keep tapping w to maintain a slightly slower speed.

This mod requires:
ScriptHook
ASI Loader
LUA Plugin

Thanks to /u/vvdevil for ideas!

Patchnotes:
V 1.0: Wrote script
V 1.1: Improved speed cap
Added option to make car invulnerable to crash damage (still vulnerable to bullets)
V 2.0:
V 2.1: Version 2.0 never happened. shhh

Source Code: hastebin.com/midubilepo.lua