Your best bet is always going to be barriers. If it's come to the point where you need to shoot through windshields to stop an attack, things are already deep in the shitter. My city hosts a lot of outdoor events in the warmer months and a few city vehicles parked at certain choke points helps mitigate anyone having the idea to plow people over with a truck .
This past year, I had the day before halloween off and my GF wanted to go to this halloween event in elgin, IL called nightmare on chicago street. It was a post-apocalyptic zombie scene that took up a few city blocks and had all sorts of people and activities inside of it. There were 4-6 entrances to the event, all on the street level and what EPD did, was use their TRT bearcats and squad cars to block off the intersections. They parked their vehicles perpendicular to the road and covered the width except for a smaller spot in the middle for people to pass in and out of. To make it all seem like part of the act, they had some squad cars with their emergency lights on.
There were thousands of people inside this event and I thought they handled it well.
I use to work for a federal agency that was highly concerned with VBIED's. All property associated with them was lined with reinforced concrete barriers, planters, and pop up steel gates that would stop a loaded dump truck. Obviously that's not feasible for your average municipality, but the practice is your best bet, I think.
