If the purpose is to disrupt travel or commerce, it would most likely achieve it's goal almost immediately. If the purpose is to bring attention to some other issue and the disruption of the highway system is unrelated (or only tangentially related), then the attention would have to be weighed against the negative impression it creates for the many affected.
I don't have the relevant experience in this area to know under what circumstances protests that negatively impact average people succeed in their goals.
In a vehicle protest you forfeit anonymity (lincense plates), ability to split since road-blocks can easily stop you, and most importantly your vehicle might be impounded.
It's the worst. You're not demonstrating anything. You're just pissing off commuters.