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Models:

Protection

The issue of “protection” can be divided in two categories which are actually either end of a continuum where the mid zone has one overlapping into the other.

Positional Protection: 

This involves some manner of creating a spatial barrier between the site improvements/buildings and moving vehicles. It does provide some level of resistance to a crashing vehicle, but this is not the primary anticipated use.

There is a “logical specification” associated with this type of barrier system where the threat analysis anticipates that “something” is required to mitigate the vulnerability of having nothing, but a major attack is not anticipated and/or the objective is to act as a deterrent.
See - Speed Control Obstacles - Test Results

Options: 

At the higher end of this positional protection is the issue of “approach” and the deflecting characteristic of vehicles impacting at oblique angles (as opposed to perpendicular) where two things occur to increase the resistance: 

1. The force of the crashing vehicle is dramatically less at oblique angles (about a fifth at 25°, half at 45° vs. 90°).
2. A sloped barrier design might deflect the crashing vehicle back into the street or cause it to roll over on impact.

Force Protection: 

This involves some manner of creating a physical barrier designed to resist the frontal attack of a crashing vehicle and not allow it to progress beyond the immediate area of the barrier. It obviously also has the traits described for positional protection previously described. 
See FM5-114 - Tested Barrier Designs

There is a “delineated specification” associated with this type of barrier where the weight and speed of the crashing vehicle are specified as well as the “allowed movement” of the barrier upon impact. 
See  DOS Specification

There is a wide range of weights and speeds based on anticipated threat and physical approach where the low end might be 2,000 lbs at 20 mph and the high end could be as high as 40,000 lbs at 50 mph.

Barriers in this category tend to be based on reinforced concrete, cast on site, including some sort of subsurface foundation. However, those that perform best have some sort of design that provides for a “progressive resistance”. 
See FM5-114 - Passive Barrier Test Results