If you are using Australian codes then the impact factor generally applies for traffic travelling at >=80km/hr. You can use direct proportion between 0-80kph. (I can't quote references off the top of my head).
Since you quote speed in mph I guess you are from the USA but similar should apply there; use direct proportion between 0-50mph.
You might refer to AASHTO for an answer; also, other codes give specific force equivalent requirements for vehicle impact, such as on barrier cables in parking structures.
The question is not quite that easy:
If the interface between the vehicle and the structure is soft/flexible, you may not need an impact factor. (For example, vehicle occupants are protected from dangerous deceleration using collapsible structures in the bumpers and chassis to increase the time over which the deceleration happens.)
Under gravity acceleration, low speed dynamic/impact loading (as force) is frequently taken as twice gravity weight. However, that factor is for sudden application of a load, and does not consider objects with a large velocity. When the object has velocity but no acceleration in the direction of impact (only deceleration), the load would be based on mass and velocity, and you would need to know the rate of deceleration to determine the force (using F=ma, or a more complex computation).
low force = mass x velocity / long deceleration time
_versus_
high force = mass x velocity / short deceleration time
For a rigid body striking a rigid structure, the force could be quite high for an instant. If the vehicle is fairly soft compared to the component being struck, loading from a slow collision is unlikely to be large.
