Sorry, I don't think I understand - are you suggesting that the freeway system itself is the target? So by staging a number of accidents simultaneously, the terrorists trap large numbers of travellers on the freeway and then attack them?
Or are you more trying to use staged incidents to funnel larger numbers of victims to a separate planned attack?
A highway is not a good location for chemical weapons usage, just because the usually-elevated road section speeds dispersal of the agent.
A mass shooting is not likely to be as deadly as Paris or Mumbai, where the police response was not as prompt as might have been hoped - freeways are good places for police to respond to, with surveillance cameras, regular highway patrols, good visibility for aerial surveillance and emergency lanes for SWAT teams to drive down.
A bigger risk is an incident at an indoor arena - basketball, ice hockey, wrestling, or a concert like in Paris, where the confined space will slow the dispersal of chemicals, the limited access to the venue will constrain the ability of police or armed forces to get to the terrorists to intervene, and fire or CS gas could cause deaths due to crushing alone.