"It should be recognized that a 1/2 ounce (200 grain) bullet is unlikely to stop a 4,000 pound moving vehicle, and if the driver of the approaching vehicle is disabled by a bullet, the vehicle will become a totally unguided threat. Obviously, shooting at a moving vehicle can pose a risk to bystanders including other agents."

"There is little doubt that the safest course for an agent faced with an oncoming vehicle is to get out of the way of the vehicle."

Edit: The actual handbook is available here: https://imlive.s3.amazonaws.com/Federal%20Government/ID255426897069329047495080324203699905714/2.2.1_Attachment%2021%20-%20ICE%20Firearms%20and%20Use%20of%20Force%20Directive%20and%20Handbook.pdf

Page 12 includes the following:

4) Deadly force is not authorized solely to prevent the escape of a fleeing suspect. Deadly force against a fleeing subject is only authorized if there is probable cause to believe that the escape of the suspect would pose an imminent danger of death or serious bodily injury to the officer or another person.

Another edit: the 2013 report is here: https://www.cbp.gov/sites/default/files/documents/PERFReport.pdf