One night in June, a driver sped through the wrong lane of traffic in Golden Gate Park in San Francisco—trying to pass another car—and killed a cyclist riding legally in the other direction. A couple of hours later , another driver in the city’s SOMA neighborhood sped through a red light and killed a 26-year-old woman on a bike.

The next day, the director of transportation at the SFMTA told a reporter that “the best bike infrastructure in the world would not have prevented these collisions.” Bike and pedestrian advocates disagree, and since the city hasn’t done much to help, they’re taking action themselves–temporarily redesigning local bike lanes themselves.

“I think many of us have recognized as pedestrians and bikers that the city is not doing very much, and when they do something, they do it in these very half-measured ways,” says one of the guerrilla bike activists, who wants to be anonymous. “So they put in some of these plastic posts and they paint the asphalt, and they say, ‘Hey, we’ve made these improvements,’ and they move on to something else.”

When the posts are run over and ignored by drivers, or the paint fades, the city hasn’t moved to more substantial, permanent solutions. A recent executive order from the mayor called for some new solutions, but advocates say the city could go much farther–and make those improvements now.

In a few recent interventions, the activists demonstrated how much better design can help. On Golden Gate Avenue, for example, a new painted bike lane was ignored by cars, and it was filled with traffic. When the activists put up simple orange construction cones, cars immediately started staying in their own lane.

“Orange cones are really a great prototyping tool,” says the activist. “They put something on the ground that’s visible. Cars really slow down around orange cones–it’s really a remarkable feat of psychology.”

The cones, inspired by groups in New York City and elsewhere that have tested similar temporary interventions, are meant to point out that bike lanes really need to be separated to be safe.