Because of improvements in macOS security, the Security & Privacy Preferences may open during the installation process. If it does, click "Allow" in order for the NVIDIA Graphics Driver to load, then return to the Installer and click "Restart".
"New Release 387.178
CUDA driver update to support CUDA Toolkit 9.1, macOS 10.13.4
and NVIDIA display driver 318.104.22.168.30.103. Supported macOS - 10.13.x"
Here's a direct download link for CUDA 387.178.
(In the past some readers said CUDA updates did work in 10.12.6 despite only listing 10.13.x support but check nvidia prefs to see if there's an update available.)
Quadro & GeForce Driver 346.03.15f13 for OS X 10.11.6 (15G20015).
(Here's a direct download link.)
The driver page text mentions a very old CUDA v7.5.30, but link goes to latest CUDA (which does not list 10.11.x support). If you already have CUDA installed, check the prefs for an update. (The last CUDA version that listed OS X 10.11.x support on the download page was CUDA 8.0.90, but the D/L page listing isn't always accurate.)