The road they blocked has an easy bypass, traffic was not significantly impacted, it is the last bit of a highway before it ends. Remember Den Hague is a seaside city, meaning that a highway going east-west is going to end there(this is the a12). During the protests my google maps suggested a 3 minute detour* if I wanted to get to Nassaubuurt (other side of the protests).

Another interesting question is the legal one, the protesters said they were planning on closing down the highway so the government closed it for them. Does that mean they even did anything unlawful?

Another bit of background: recently, Dutch farmers closed down almost all highways by driving their tractors on them and dumping trash on them to protest environmental regulations. Almost nobody was arrested for that. However, shortly before these climate demonstrations most of the organizers were arrested. This greatly increased the size of these demonstrations as they were joined by oxfam novib and others because it was a great example of how climate protestors are punished far harder than industrial/commercial-based protests.

*Assuming of course that you aren't willing to take public transport as the region has excellent accessibility by train/tram/bus (the whole thing happened in sight of Den Hague central station.)