IMHO (not a lawyer obviously) but you not know where it came from other than the vehicle in front of you would absolve them of liability. Even in the case it came out of the sand/salt spreader, if it were me I'd take my L and just stay further back next time. Those thinga whole job is to shoot sand/salt out the back. An unsecured load of debris is different because there is a presumed responsibility of the drover to secure their load and maintain the safety for others