diff --git a/headless/src/main.rs b/headless/src/main.rs
index c4b4cd73a9..d50ec89956 100644
--- a/headless/src/main.rs
+++ b/headless/src/main.rs
@@ -29,7 +29,7 @@ struct Flags {
     big_sim: bool,
 
     /// Scenario name for savestating
-    #[structopt(long = "scenario_name", default_value = "editor")]
+    #[structopt(long = "scenario_name", default_value = "headless")]
     scenario_name: String,
 }
 
@@ -62,15 +62,15 @@ fn main() {
         None
     };
 
-    let mut benchmark = sim.start_benchmark();
-    loop {
-        sim.step(&map, &control_map);
-        if sim.time.is_multiple_of(sim::Tick::from_seconds(60)) {
-            let speed = sim.measure_speed(&mut benchmark);
-            println!("{0}, speed = {1:.2}x", sim.summary(), speed);
-        }
-        if Some(sim.time) == save_at {
-            sim.save();
-        }
-    }
+    sim::init::run_until_done(
+        &mut sim,
+        &map,
+        &control_map,
+        |sim| {
+            if Some(sim.time) == save_at {
+                sim.save();
+            }
+        },
+        |_parked| {},
+    );
 }
diff --git a/sim/src/init.rs b/sim/src/init.rs
index 4138010f6d..604c8be227 100644
--- a/sim/src/init.rs
+++ b/sim/src/init.rs
@@ -1,7 +1,7 @@
 use abstutil;
 use control::ControlMap;
 use map_model::{Edits, Map};
-use {Sim, Tick};
+use {ParkedCar, Sim, Tick};
 
 // Convenience method to setup everything.
 pub fn load(
@@ -45,14 +45,24 @@ pub fn big_spawn(sim: &mut Sim, map: &Map) {
 // TODO share the helpers for spawning specific parking spots and stuff?
 
 // TODO time limit and a callback for the step results?
-pub fn run_until_done(sim: &mut Sim, map: &Map, control_map: &ControlMap) {
+pub fn run_until_done<CB1, CB2>(
+    sim: &mut Sim,
+    map: &Map,
+    control_map: &ControlMap,
+    sim_cb: CB1,
+    handle_step: CB2,
+) where
+    CB1: Fn(&Sim),
+    CB2: Fn(Vec<ParkedCar>),
+{
     let mut benchmark = sim.start_benchmark();
     loop {
-        sim.step(&map, &control_map);
+        handle_step(sim.step(&map, &control_map));
         if sim.time.is_multiple_of(Tick::from_seconds(60)) {
             let speed = sim.measure_speed(&mut benchmark);
             println!("{0}, speed = {1:.2}x", sim.summary(), speed);
         }
+        sim_cb(sim);
         if sim.is_done() {
             break;
         }
diff --git a/sim/tests/completion.rs b/sim/tests/completion.rs
index 667a7bb167..45d3e01d74 100644
--- a/sim/tests/completion.rs
+++ b/sim/tests/completion.rs
@@ -12,7 +12,7 @@ fn aorta_model_completes() {
         Some(sim::Tick::from_seconds(30)),
     );
     sim::init::small_spawn(&mut sim, &map);
-    sim::init::run_until_done(&mut sim, &map, &control_map);
+    sim::init::run_until_done(&mut sim, &map, &control_map, |_sim| {}, |_parked| {});
 }
 
 // TODO other tests (not completion) to add:
diff --git a/sim/tests/parking.rs b/sim/tests/parking.rs
index 2bd63a2a4f..b6f4b973ed 100644
--- a/sim/tests/parking.rs
+++ b/sim/tests/parking.rs
@@ -20,19 +20,19 @@ fn park_on_goal_st() {
     sim.seed_specific_parked_cars(parking2, (5..8).collect());
     sim.start_parked_car_with_goal(&map, car, driving2);
 
-    loop {
-        if let Some(p) = sim.step(&map, &control_map).first() {
-            assert_eq!(p.car, car);
-            assert_eq!(p.spot.lane, parking2);
-            assert_eq!(p.spot.idx, 4);
-            break;
-        }
-        if sim.time.is_multiple_of(sim::Tick::from_seconds(60)) {
-            println!("{}", sim.summary());
-        }
-        // TODO time limit
-    }
-    println!("Expected conditions met at {}", sim.time);
+    sim::init::run_until_done(
+        &mut sim,
+        &map,
+        &control_map,
+        |_sim| {},
+        |parked| {
+            if let Some(p) = parked.first() {
+                assert_eq!(p.car, car);
+                assert_eq!(p.spot.lane, parking2);
+                assert_eq!(p.spot.idx, 4);
+            }
+        },
+    );
 }
 
 #[test]
@@ -48,19 +48,19 @@ fn wander_around_for_parking() {
     sim.seed_specific_parked_cars(parking2, (0..8).collect());
     sim.start_parked_car_with_goal(&map, car, driving2);
 
-    loop {
-        if let Some(p) = sim.step(&map, &control_map).first() {
-            assert_eq!(p.car, car);
-            assert_eq!(p.spot.lane, parking1);
-            assert_eq!(p.spot.idx, 0);
-            break;
-        }
-        if sim.time.is_multiple_of(sim::Tick::from_seconds(60)) {
-            println!("{}", sim.summary());
-        }
-        // TODO time limit
-    }
-    println!("Expected conditions met at {}", sim.time);
+    sim::init::run_until_done(
+        &mut sim,
+        &map,
+        &control_map,
+        |_sim| {},
+        |parked| {
+            if let Some(p) = parked.first() {
+                assert_eq!(p.car, car);
+                assert_eq!(p.spot.lane, parking1);
+                assert_eq!(p.spot.idx, 0);
+            }
+        },
+    );
 }
 
 fn setup(