diff --git a/osm2streets/src/operations/mod.rs b/osm2streets/src/operations/mod.rs
index c7b5bc35..8c174c75 100644
--- a/osm2streets/src/operations/mod.rs
+++ b/osm2streets/src/operations/mod.rs
@@ -2,3 +2,4 @@
 
 mod collapse_intersection;
 mod collapse_short_road;
+mod update_geometry;
diff --git a/osm2streets/src/operations/update_geometry.rs b/osm2streets/src/operations/update_geometry.rs
new file mode 100644
index 00000000..42d425be
--- /dev/null
+++ b/osm2streets/src/operations/update_geometry.rs
@@ -0,0 +1,66 @@
+use geom::{Circle, Distance};
+
+use crate::{IntersectionID, Road, StreetNetwork};
+
+impl StreetNetwork {
+    /// Recalculates trim distances and intersection geometry. This is idempotent; it doesn't use
+    /// any results from the previous call.
+    pub fn update_geometry(&mut self, id: IntersectionID) {
+        let i = &self.intersections[&id];
+
+        // Update the polygon and Set trim distances for roads
+        let input_roads = i
+            .roads
+            .iter()
+            .map(|r| self.roads[r].to_input_road(self.config.driving_side))
+            .collect::<Vec<_>>();
+        match crate::intersection_polygon(i.id, input_roads, &i.trim_roads_for_merging) {
+            Ok(results) => {
+                self.intersections.get_mut(&id).unwrap().polygon = results.intersection_polygon;
+
+                for (r, dist) in results.trim_starts {
+                    self.roads.get_mut(&r).unwrap().trim_start = dist;
+                }
+                for (r, dist) in results.trim_ends {
+                    self.roads.get_mut(&r).unwrap().trim_end = dist;
+                }
+                for (pt, label) in results.debug {
+                    self.debug_point(pt, label);
+                }
+            }
+            Err(err) => {
+                error!("Can't make intersection geometry for {}: {}", i.id, err);
+
+                let r = i.roads[0];
+                // Don't trim lines back at all
+                let road = &self.roads[&r];
+                let pt = if road.src_i == i.id {
+                    road.center_line.first_pt()
+                } else {
+                    road.center_line.last_pt()
+                };
+                self.intersections.get_mut(&id).unwrap().polygon =
+                    Circle::new(pt, Distance::meters(3.0)).to_polygon();
+            }
+        }
+
+        // Update road center lines based on the trim. Note update_geometry works on one
+        // intersection at a time, so it's possible some roads haven't been trimmed yet on the
+        // other side.
+        for r in &self.intersections[&id].roads {
+            let road = self.roads.get_mut(r).unwrap();
+
+            let untrimmed = road.get_untrimmed_center_line(self.config.driving_side);
+            if let Some(pl) =
+                Road::trim_polyline_both_ends(untrimmed.clone(), road.trim_start, road.trim_end)
+            {
+                road.center_line = pl;
+            } else {
+                error!("{} got trimmed into oblivion, collapse it later", road.id);
+                road.center_line = untrimmed;
+                // Collapse it later
+                road.internal_junction_road = true;
+            }
+        }
+    }
+}
diff --git a/osm2streets/src/transform/intersection_geometry.rs b/osm2streets/src/transform/intersection_geometry.rs
index 70e70b75..b66a9d69 100644
--- a/osm2streets/src/transform/intersection_geometry.rs
+++ b/osm2streets/src/transform/intersection_geometry.rs
@@ -1,68 +1,16 @@
 use abstutil::Timer;
-use geom::{Circle, Distance};
 
-use crate::{Road, StreetNetwork};
+use crate::StreetNetwork;
 
 pub fn generate(streets: &mut StreetNetwork, timer: &mut Timer) {
     timer.start_iter(
         "find each intersection polygon",
         streets.intersections.len(),
     );
-    // It'd be nice to mutate in the loop, but the borrow checker won't let us
-    let mut set_polygons = Vec::new();
 
-    // Set trim distances for all roads
-    for i in streets.intersections.values() {
+    let ids = streets.intersections.keys().cloned().collect::<Vec<_>>();
+    for i in ids {
         timer.next();
-        let input_roads = i
-            .roads
-            .iter()
-            .map(|r| streets.roads[r].to_input_road(streets.config.driving_side))
-            .collect::<Vec<_>>();
-        match crate::intersection_polygon(i.id, input_roads, &i.trim_roads_for_merging) {
-            Ok(results) => {
-                set_polygons.push((i.id, results.intersection_polygon));
-                for (r, dist) in results.trim_starts {
-                    streets.roads.get_mut(&r).unwrap().trim_start = dist;
-                }
-                for (r, dist) in results.trim_ends {
-                    streets.roads.get_mut(&r).unwrap().trim_end = dist;
-                }
-                for (pt, label) in results.debug {
-                    streets.debug_point(pt, label);
-                }
-            }
-            Err(err) => {
-                error!("Can't make intersection geometry for {}: {}", i.id, err);
-
-                let r = i.roads[0];
-                // Don't trim lines back at all
-                let road = &streets.roads[&r];
-                let pt = if road.src_i == i.id {
-                    road.center_line.first_pt()
-                } else {
-                    road.center_line.last_pt()
-                };
-                set_polygons.push((i.id, Circle::new(pt, Distance::meters(3.0)).to_polygon()));
-            }
-        }
-    }
-
-    for road in streets.roads.values_mut() {
-        let untrimmed = road.get_untrimmed_center_line(streets.config.driving_side);
-        if let Some(pl) =
-            Road::trim_polyline_both_ends(untrimmed.clone(), road.trim_start, road.trim_end)
-        {
-            road.center_line = pl;
-        } else {
-            error!("{} got trimmed into oblivion, collapse it later", road.id);
-            road.center_line = untrimmed;
-            // Collapse it later
-            road.internal_junction_road = true;
-        }
-    }
-
-    for (i, polygon) in set_polygons {
-        streets.intersections.get_mut(&i).unwrap().polygon = polygon;
+        streets.update_geometry(i);
     }
 }