diff --git a/game/src/app.rs b/game/src/app.rs
index 356cec58e3..e06a2206c3 100644
--- a/game/src/app.rs
+++ b/game/src/app.rs
@@ -43,6 +43,7 @@ impl App {
         ctx.set_style(cs.gui_style.clone());
 
         let primary = ctx.loading_screen("load map", |ctx, mut timer| {
+            assert!(flags.sim_flags.modifiers.is_empty());
             let (map, sim, _) = flags.sim_flags.load(timer);
             PerMap::map_loaded(map, sim, splash, flags, &opts, &cs, ctx, &mut timer)
         });
diff --git a/game/src/lib.rs b/game/src/lib.rs
index 4dce94d393..c707e84eef 100644
--- a/game/src/lib.rs
+++ b/game/src/lib.rs
@@ -3,7 +3,7 @@ extern crate log;
 
 use abstutil::{CmdArgs, Timer};
 use geom::Duration;
-use sim::{ScenarioModifier, SimFlags};
+use sim::SimFlags;
 
 use crate::app::Flags;
 
@@ -104,6 +104,11 @@ pub fn main() {
             sandbox::TutorialPointer::new(n - 1, 0),
         ));
     }
+
+    // Don't keep the scenario modifiers in the original sim_flags; they shouldn't apply to
+    // other scenarios loaed in the UI later.
+    let modifiers = flags.sim_flags.modifiers.drain(..).collect();
+
     if mode.is_none() && flags.sim_flags.load.contains("scenarios/") {
         // TODO regex
         let parts = flags.sim_flags.load.split("/").collect::<Vec<_>>();
@@ -111,12 +116,6 @@ pub fn main() {
         let scenario = abstutil::basename(parts[parts.len() - 1]);
         flags.sim_flags.load = abstutil::path_map(&map_name);
 
-        let modifiers: Vec<ScenarioModifier> = args
-            .optional_parse("--scenario_modifiers", |s| {
-                abstutil::from_json(&s.to_string().into_bytes())
-            })
-            .unwrap_or_else(Vec::new);
-
         mode = Some(sandbox::GameplayMode::PlayScenario(
             map_name, scenario, modifiers,
         ));
diff --git a/sim/src/make/load.rs b/sim/src/make/load.rs
index aecabc71e7..0903a002e3 100644
--- a/sim/src/make/load.rs
+++ b/sim/src/make/load.rs
@@ -26,11 +26,16 @@ impl SimFlags {
         let rng_seed = args
             .optional_parse("--rng_seed", |s| s.parse())
             .unwrap_or(SimFlags::RNG_SEED);
+        let modifiers: Vec<ScenarioModifier> = args
+            .optional_parse("--scenario_modifiers", |s| {
+                abstutil::from_json(&s.to_string().into_bytes())
+            })
+            .unwrap_or_else(Vec::new);
         SimFlags {
             load: args
                 .optional_free()
                 .unwrap_or_else(|| abstutil::path_map("montlake")),
-            modifiers: Vec::new(),
+            modifiers,
             rng_seed,
             opts: SimOptions::from_args(args, rng_seed),
         }