Impact is based on span length not speed using AASHTO Standard Specs.
The LRFD spec calls impact something different and I am not sure it is related to speed or weight (I have done limited LRFD design) .
Different states may have requirements that take this into account but you would need to refer to their specific design standard.
AASHTO LRFD refers to impact as Dynamic Load Allowance(Section 3.6.2) The allowance is taken as 1.33 although 1.15 is used for fatigue and certain limit states and 1.75 for the design of joints.
The AASHTO Standard Specs based impact on span length with a maximum value of 30%. In reality, impact is due to the motion of the truck and is based on a number of factors such as speed, structure stiffness, friction between wheels and wearing surface.
I've heard it argued that if a truck travels at 5 to 10 mph there is little or no impact effect.
Umassengr-
LRFD spec calls it a dynamic load allowance. Deck joints have a DLA of 75%, fatigue and fracture limit states have a DLA of 15% and all other limit states have a DLA of 33%.
"Do we have any different impact factor for OFF-highway truck bridges, Like state routes etc"
If you are using AASHTO, then the answer is no. Check your state bridge design manual. They may have a modification to AASHTO, but I doubt it.
You may be able to use a lighter truck than that required for interstate routes, which would indirectly reduce you impact loading. The impact factor would of couse stay the same.
AASHTO's Manual for Condition Evaluation of Bridges allows for a reduction of impact "when conditions of alignment, engorced speed posting, and similar situations require a vehicle to substantially reduce speed" (6.7.4). But you should keep in mind that this manual is for inspection and rating of existing structures, not the design of new structures.
The only other case I'm aware of when impact factors can be reduced is when you are dealing with a unique loading. Many jurisdictions will allow for a reduced impact factor for one time loads as long as an engineer defines the procedure. This generally requires road closure for all other traffic and specifies that no braking or changing of gears is allowed while on the bridge.
