Where to install OpenCV (which directory, NOT from apt-get package)?

relatively new Ubuntu user here (but very familiar with OpenCV use on Windows). Having read this post:

What is the Linux equivalent to Windows' Program Files?

It's clear that there is not a direct Ubuntu/Linux equivalent of "Program Files" or "Program Files (x86)" in Windows.

For OpenCV installation, I followed this page:

http://milq.github.io/install-opencv-ubuntu-debian/

which essentially involves going to here:

https://github.com/milq/milq/tree/master/scripts/bash

and running 3 scripts:

install-additional-cli-software.sh

install-additional-gui-software.sh

install-opencv.sh

This works like a champ with Ubuntu 16.04, however I'm not clear on which directory to perform these steps from and therefore have the install located in.

If I open up a terminal by default it goes to the home directory for the current user, in my case for example that would be "/home/cdahms"

This seems like a very bad place to locate an OpenCV download/build/install (still not sure which to call it, again, relatively new to Linux) for at least the following reasons:

-The home directory is often used for other stuff and locating OpenCV here may result in inadvertent modification at a later time
-If a computer ever has multiple users it does not seem to make sense to place OpenCV in a certain user's home directory
-A home directory does not seem like a proper directory for a software install generally speaking

Based on this:

https://github.com/opencv/opencv/blob/master/CMakeLists.txt

line 41:

set(CMAKE_INSTALL_PREFIX "/usr/local" CACHE PATH "Installation Directory")

I'm under the impression that I should cd to "/usr/local", then run the above OpenCV install script. Is this correct? Where are most Ubuntu/OpenCV users installing to?

Note: I should mention that I'm NOT using the apt-get package "libopencv-dev" since that is on OpenCV version 2.4.9 currently, which is about 3.5 years old now. Does anybody have any ideas as to why this package is not updated more often?