Ensure means to make sure or certain that X will occur. Insure means to provide assurance that Y will occur if X does.
Example of proper usage
The cropped image shows an example of correct word selection. This example describes the duties of traffic engineers, reporting that “they ensure engineering principles are used”. This usage is correct.
Example of improper usage
Using insure in that example would result in the following:
However, the usage seen in the image makes very good sense. It means that traffic engineers make sure or certain that engineering principles are used.
Rule of thumb
Here is a mnemonic device that I use to distinguish between the two words.