diff --git a/sim/src/driving.rs b/sim/src/driving.rs
index a863884348..ddb63c52d0 100644
--- a/sim/src/driving.rs
+++ b/sim/src/driving.rs
@@ -7,6 +7,7 @@ use geom::{Angle, Pt2D};
 use intersections::{IntersectionPolicy, StopSign, TrafficSignal};
 use map_model::{LaneID, LaneType, Map, TurnID};
 use multimap::MultiMap;
+use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use std;
 use std::collections::{BTreeMap, HashSet, VecDeque};
 use std::f64;
@@ -214,10 +215,33 @@ pub struct DrivingSimState {
     // Using BTreeMap instead of HashMap so iteration is deterministic.
     pub(crate) cars: BTreeMap<CarID, Car>,
     lanes: Vec<SimQueue>,
+    // See https://github.com/serde-rs/json/issues/402.
+    #[serde(serialize_with = "serialize_turn_map")]
+    #[serde(deserialize_with = "deserialize_turn_map")]
     turns: BTreeMap<TurnID, SimQueue>,
     intersections: Vec<IntersectionPolicy>,
 }
 
+fn serialize_turn_map<S: Serializer>(
+    map: &BTreeMap<TurnID, SimQueue>,
+    s: S,
+) -> Result<S::Ok, S::Error> {
+    map.iter()
+        .map(|(a, b)| (a.clone(), b.clone()))
+        .collect::<Vec<(_, _)>>()
+        .serialize(s)
+}
+fn deserialize_turn_map<'de, D: Deserializer<'de>>(
+    d: D,
+) -> Result<BTreeMap<TurnID, SimQueue>, D::Error> {
+    let vec = <Vec<(TurnID, SimQueue)>>::deserialize(d)?;
+    let mut map = BTreeMap::new();
+    for (k, v) in vec {
+        map.insert(k, v);
+    }
+    Ok(map)
+}
+
 impl DrivingSimState {
     pub fn new(map: &Map) -> DrivingSimState {
         let mut intersections: Vec<IntersectionPolicy> = Vec::new();
diff --git a/sim/src/walking.rs b/sim/src/walking.rs
index cf52bc7fab..e50be15c07 100644
--- a/sim/src/walking.rs
+++ b/sim/src/walking.rs
@@ -3,6 +3,7 @@ use dimensioned::si;
 use draw_ped::DrawPedestrian;
 use map_model::{Lane, LaneID, Map, Turn, TurnID};
 use multimap::MultiMap;
+use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use std;
 use std::collections::VecDeque;
 use {On, PedestrianID};
@@ -105,11 +106,34 @@ impl Pedestrian {
 pub struct WalkingSimState {
     // Trying a different style than driving for storing things
     peds_per_sidewalk: MultiMap<LaneID, Pedestrian>,
+    #[serde(serialize_with = "serialize_turn_map")]
+    #[serde(deserialize_with = "deserialize_turn_map")]
     peds_per_turn: MultiMap<TurnID, Pedestrian>,
 
     id_counter: usize,
 }
 
+fn serialize_turn_map<S: Serializer>(
+    map: &MultiMap<TurnID, Pedestrian>,
+    s: S,
+) -> Result<S::Ok, S::Error> {
+    // TODO maybe need to sort by TurnID to have deterministic output
+    map.iter()
+        .map(|(a, b)| (a.clone(), b.clone()))
+        .collect::<Vec<(_, _)>>()
+        .serialize(s)
+}
+fn deserialize_turn_map<'de, D: Deserializer<'de>>(
+    d: D,
+) -> Result<MultiMap<TurnID, Pedestrian>, D::Error> {
+    let vec = <Vec<(TurnID, Pedestrian)>>::deserialize(d)?;
+    let mut map = MultiMap::new();
+    for (k, v) in vec {
+        map.insert(k, v);
+    }
+    Ok(map)
+}
+
 impl WalkingSimState {
     pub fn new() -> WalkingSimState {
         WalkingSimState {
diff --git a/sim/tests/determinism.rs b/sim/tests/determinism.rs
index 36e13a2d96..cdca8874cc 100644
--- a/sim/tests/determinism.rs
+++ b/sim/tests/determinism.rs
@@ -3,6 +3,16 @@ extern crate control;
 extern crate map_model;
 extern crate sim;
 
+#[test]
+fn serialization() {
+    // This assumes this map has been built
+    let input = "../data/small.abst";
+
+    let map = map_model::Map::new(input, &map_model::Edits::new()).expect("Couldn't load map");
+    let sim = sim::Sim::new(&map, Some(42));
+    abstutil::write_json("/tmp/sim_state.json", &sim).unwrap();
+}
+
 #[test]
 fn from_scratch() {
     // This assumes this map has been built
@@ -11,8 +21,6 @@ fn from_scratch() {
     let spawn_count = 1000;
 
     println!("Creating two simulations");
-    // TODO bundle all of the layers of the map together in some super-struct, so this
-    // initialization and plumbing is easier
     let map = map_model::Map::new(input, &map_model::Edits::new()).expect("Couldn't load map");
     let control_map = control::ControlMap::new(&map);