Restore old logic partly. Now SMP gridlocks horribly.

sim/src/mechanics/intersection.rs

@@ -864,6 +864,8 @@ impl IntersectionSimState {
             return false;
         }
 
+        let turn = map.get_t(req.turn);
+
         // Once upon a time, we'd make sure that this request doesn't conflict with another in
         // self.waiting:
         // 1) Higher-ranking turns get to go first.
@@ -876,11 +878,21 @@ impl IntersectionSimState {
         // that events are processed in time order mean that case #2 is magically handled anyway.
         // If a case #1 could've started by now, then they would have. Since they didn't, they must
         // be blocked.
+        for other_req in self.state[&req.turn.parent].waiting.keys() {
+            if map.get_t(other_req.turn).conflicts_with(turn)
+                && our_priority < sign.get_priority(other_req.turn, map)
+            {
+                info!(
+                    "{} yielding to {} who's waiting",
+                    req.agent, other_req.agent
+                );
+                return false;
+            }
+        }
 
         // Can we optimistically finish this turn before an approaching higher-priority vehicle
         // wants to begin?
 
-        let turn = map.get_t(req.turn);
         let expected_finish = now + turn.geom.length() / speed;
         for (_, (other_req, eta)) in &self.state[&req.turn.parent].leader_eta {
             if expected_finish < *eta {