Roundabouts are unlocked later in the game. They are incredibly powerful for unblocked traffic flow, but only if you follow the rule: Do not place intersections too close to the roundabout.
If an intersection is one tile away from a roundabout, cars will stop in the roundabout to wait for the intersection. Keep a 4-tile buffer zone.
Dinosaur Polo Club occasionally releases a limited-time WebGL demo for browsers. While the full game isn't free, the demo offers a taste of the first two maps (Los Angeles and Beijing). Because it runs on standard HTML5, most school filters do not block it because it isn't categorized as a "gambling" or "MMO" site.
If you need to get your traffic fix right now, here are the safest and most effective methods to play unblocked.