diff --git a/docs/dev.md b/docs/dev.md
index 1918c92ec4..5504d6a1ab 100644
--- a/docs/dev.md
+++ b/docs/dev.md
@@ -42,9 +42,6 @@ One-time setup:
     experience, so they're hidden for now.
   - `cargo run -- --tutorial=12` starts somewhere in the tutorial
   - Adding `--edits='name of edits'` starts with edits applied to the map.
-- If you're testing anything related to prebaked results (used for comparisons
-  against a baseline in challenge mode), make sure to set `--rng_seed=42`. The
-  `--dev` flag does by default.
 - All code is automatically formatted using
   https://github.com/rust-lang/rustfmt; please run `cargo +nightly fmt` before
   sending a PR. (You have to install the nightly toolchain just for fmt)
diff --git a/game/src/abtest/setup.rs b/game/src/abtest/setup.rs
index d75a917b15..b1b54632b6 100644
--- a/game/src/abtest/setup.rs
+++ b/game/src/abtest/setup.rs
@@ -111,9 +111,6 @@ fn launch_test(test: &ABTest, app: &mut App, ctx: &mut EventCtx) -> ABTestMode {
 
             {
                 timer.start("load primary");
-                if app.primary.current_flags.sim_flags.rng_seed.is_none() {
-                    app.primary.current_flags.sim_flags.rng_seed = Some(42);
-                }
                 app.primary.current_flags.sim_flags.opts.run_name =
                     format!("{} with {}", test.test_name, test.edits1_name);
                 app.primary.current_flags.sim_flags.opts.savestate_every = None;
diff --git a/game/src/main.rs b/game/src/main.rs
index adc2feae3c..d5fcd0b0c6 100644
--- a/game/src/main.rs
+++ b/game/src/main.rs
@@ -36,16 +36,7 @@ fn main() {
         num_agents: args.optional_parse("--num_agents", |s| s.parse()),
     };
     let mut opts = options::Options::default();
-    if args.enabled("--dev") {
-        opts.dev = true;
-        flags.sim_flags.rng_seed = Some(42);
-    }
-
-    // No random in wasm
-    #[cfg(target_arch = "wasm32")]
-    {
-        flags.sim_flags.rng_seed = Some(42);
-    }
+    opts.dev = args.enabled("--dev");
 
     if let Some(x) = args.optional("--color_scheme") {
         let mut ok = false;
diff --git a/game/src/sandbox/gameplay/tutorial.rs b/game/src/sandbox/gameplay/tutorial.rs
index a70558ace1..cbacd4a0be 100644
--- a/game/src/sandbox/gameplay/tutorial.rs
+++ b/game/src/sandbox/gameplay/tutorial.rs
@@ -790,10 +790,7 @@ impl TutorialState {
         // TODO Should some of this always happen?
         app.primary.clear_sim();
         if let Some(ref cb) = self.stage().spawn {
-            let old = app.primary.current_flags.sim_flags.rng_seed;
-            app.primary.current_flags.sim_flags.rng_seed = Some(42);
             (cb)(app);
-            app.primary.current_flags.sim_flags.rng_seed = old;
             app.primary
                 .sim
                 .normal_step(&app.primary.map, Duration::seconds(0.1));
diff --git a/headless/src/main.rs b/headless/src/main.rs
index eb3ebaec0d..1cb233b7c5 100644
--- a/headless/src/main.rs
+++ b/headless/src/main.rs
@@ -15,7 +15,7 @@ fn main() {
     args.done();
 
     let mut sim_flags = SimFlags::synthetic_test("montlake", "pandemic");
-    sim_flags.opts.enable_pandemic_model = Some(XorShiftRng::from_seed([42; 16]));
+    sim_flags.opts.enable_pandemic_model = Some(XorShiftRng::from_seed([sim_flags.rng_seed; 16]));
     let mut timer = Timer::new("setup headless");
     let (map, mut sim, mut rng) = sim_flags.load(&mut timer);
 
diff --git a/release/play_abstreet.bat b/release/play_abstreet.bat
index 894494fe58..7dfb6a6a53 100755
--- a/release/play_abstreet.bat
+++ b/release/play_abstreet.bat
@@ -1,3 +1,3 @@
 cd game
 set RUST_BACKTRACE=1
-game.exe --rng_seed=42 1> ..\\output.txt 2>&1
+game.exe 1> ..\\output.txt 2>&1
diff --git a/release/play_abstreet.sh b/release/play_abstreet.sh
index eab7694571..92409908dc 100755
--- a/release/play_abstreet.sh
+++ b/release/play_abstreet.sh
@@ -1,4 +1,4 @@
 #!/bin/bash
 
 cd game
-RUST_BACKTRACE=1 ./game --rng_seed=42
+RUST_BACKTRACE=1 ./game
diff --git a/sim/src/make/load.rs b/sim/src/make/load.rs
index 2a584ff85c..5ea3ab8ad6 100644
--- a/sim/src/make/load.rs
+++ b/sim/src/make/load.rs
@@ -5,17 +5,21 @@ use map_model::{Map, MapEdits};
 use rand::SeedableRng;
 use rand_xorshift::XorShiftRng;
 
+const RNG_SEED: u8 = 42;
+
 #[derive(Clone)]
 pub struct SimFlags {
     pub load: String,
     pub use_map_fixes: bool,
-    pub rng_seed: Option<u8>,
+    pub rng_seed: u8,
     pub opts: SimOptions,
 }
 
 impl SimFlags {
     pub fn from_args(args: &mut CmdArgs) -> SimFlags {
-        let rng_seed = args.optional_parse("--rng_seed", |s| s.parse());
+        let rng_seed = args
+            .optional_parse("--rng_seed", |s| s.parse())
+            .unwrap_or(RNG_SEED);
 
         SimFlags {
             load: args
@@ -33,11 +37,7 @@ impl SimFlags {
                 recalc_lanechanging: !args.enabled("--dont_recalc_lc"),
                 break_turn_conflict_cycles: args.enabled("--break_turn_conflict_cycles"),
                 enable_pandemic_model: if args.enabled("--pandemic") {
-                    if let Some(seed) = rng_seed {
-                        Some(XorShiftRng::from_seed([seed; 16]))
-                    } else {
-                        Some(XorShiftRng::from_entropy())
-                    }
+                    Some(XorShiftRng::from_seed([rng_seed; 16]))
                 } else {
                     None
                 },
@@ -63,17 +63,13 @@ impl SimFlags {
         SimFlags {
             load: abstutil::path_map(map),
             use_map_fixes: true,
-            rng_seed: Some(42),
+            rng_seed: RNG_SEED,
             opts: SimOptions::new(run_name),
         }
     }
 
     pub fn make_rng(&self) -> XorShiftRng {
-        if let Some(seed) = self.rng_seed {
-            XorShiftRng::from_seed([seed; 16])
-        } else {
-            XorShiftRng::from_entropy()
-        }
+        XorShiftRng::from_seed([self.rng_seed; 16])
     }
 
     // Convenience method to setup everything.