diff --git a/backend/src/fix_osm.rs b/backend/src/fix_osm.rs
new file mode 100644
index 0000000..7b2efed
--- /dev/null
+++ b/backend/src/fix_osm.rs
@@ -0,0 +1,38 @@
+use geo::Intersects;
+use geojson::GeoJson;
+use rstar::{RTree, RTreeObject};
+
+use crate::MapModel;
+
+pub fn find_separate_sidewalks(map: &MapModel) -> GeoJson {
+    let footways = RTree::bulk_load(
+        map.roads
+            .iter()
+            .filter(|r| r.tags.is("highway", "footway"))
+            .map(|r| r.linestring.clone())
+            .collect(),
+    );
+
+    let mut features = Vec::new();
+    'ROAD: for r in &map.roads {
+        if r.tags.is("highway", "footway")
+            || r.tags.is("sidewalk", "separate")
+            || r.tags.is("sidewalk:left", "separate")
+            || r.tags.is("sidewalk:right", "separate")
+            || r.tags.is("sidewalk:both", "separate")
+        {
+            continue;
+        }
+
+        // Along this road, if we project away, do we hit a footway?
+        for line in crate::heatmap::make_perpendicular_offsets(&r.linestring, 25.0, 15.0) {
+            for footway in footways.locate_in_envelope_intersecting(&line.envelope()) {
+                if footway.intersects(&line) {
+                    features.push(r.to_gj(&map.mercator));
+                    continue 'ROAD;
+                }
+            }
+        }
+    }
+    GeoJson::from(features)
+}
diff --git a/backend/src/heatmap.rs b/backend/src/heatmap.rs
index 593e8a2..ad1e3e4 100644
--- a/backend/src/heatmap.rs
+++ b/backend/src/heatmap.rs
@@ -97,7 +97,7 @@ fn calculate(map: &mut MapModel, requests: Vec<CompareRouteRequest>) -> FeatureC
 }
 
 // TODO canvas_geometry needs this too
-fn make_perpendicular_offsets(
+pub fn make_perpendicular_offsets(
     linestring: &LineString,
     walk_every_m: f64,
     project_away_m: f64,
diff --git a/backend/src/lib.rs b/backend/src/lib.rs
index ca15598..73596ad 100644
--- a/backend/src/lib.rs
+++ b/backend/src/lib.rs
@@ -14,6 +14,7 @@ use wasm_bindgen::prelude::*;
 
 use crate::profiles::Profile;
 
+mod fix_osm;
 mod heatmap;
 mod profiles;
 mod route;
@@ -174,6 +175,13 @@ impl MapModel {
         vec![b.min().x, b.min().y, b.max().x, b.max().y]
     }
 
+    #[wasm_bindgen(js_name = findSeparateSidewalks)]
+    pub fn find_separate_sidewalks(&self) -> Result<String, JsValue> {
+        let out =
+            serde_json::to_string(&fix_osm::find_separate_sidewalks(self)).map_err(err_to_js)?;
+        Ok(out)
+    }
+
     fn find_edge(&self, i1: IntersectionID, i2: IntersectionID) -> &Road {
         // TODO Store lookup table
         for r in &self.intersections[i1.0].roads {
diff --git a/web/src/App.svelte b/web/src/App.svelte
index 742ecb7..1b43bbf 100644
--- a/web/src/App.svelte
+++ b/web/src/App.svelte
@@ -12,6 +12,7 @@
   import DebugMode from "./DebugMode.svelte";
   import RouteMode from "./RouteMode.svelte";
   import ScoreMode from "./ScoreMode.svelte";
+  import OsmSeparateSidewalksMode from "./OsmSeparateSidewalksMode.svelte";
   import NetworkLayer from "./NetworkLayer.svelte";
   import {
     map as mapStore,
@@ -96,31 +97,33 @@
       <hr />
       <div><button on:click={zoomToFit}>Zoom to fit</button></div>
 
-      <Legend
-        rows={[
-          ["Footway (ground, outdoors)", kindToColor.Footway],
-          ["Indoors footway", kindToColor.Indoors],
-          ["Footway not on the ground", kindToColor.BridgeOrTunnel],
-          [
-            "Street with vehicle traffic (maybe with a sidewalk, maybe not)",
-            kindToColor.WithTraffic,
-          ],
-          ["Crossing", kindToColor.Crossing],
-          ["Severance", kindToColor.Severance],
-        ]}
-      />
-      <div>
-        <label>
-          <input type="checkbox" bind:checked={showSeverances} />
-          Show severances
-        </label>
-      </div>
-      <div>
-        <label>
-          Network opacity:
-          <input type="range" min="0" max="100" bind:value={opacity} />
-        </label>
-      </div>
+      {#if $mode.kind != "osm-separate-sidewalks"}
+        <Legend
+          rows={[
+            ["Footway (ground, outdoors)", kindToColor.Footway],
+            ["Indoors footway", kindToColor.Indoors],
+            ["Footway not on the ground", kindToColor.BridgeOrTunnel],
+            [
+              "Street with vehicle traffic (maybe with a sidewalk, maybe not)",
+              kindToColor.WithTraffic,
+            ],
+            ["Crossing", kindToColor.Crossing],
+            ["Severance", kindToColor.Severance],
+          ]}
+        />
+        <div>
+          <label>
+            <input type="checkbox" bind:checked={showSeverances} />
+            Show severances
+          </label>
+        </div>
+        <div>
+          <label>
+            Network opacity:
+            <input type="range" min="0" max="100" bind:value={opacity} />
+          </label>
+        </div>
+      {/if}
     {/if}
   </div>
   <div slot="main" style="position:relative; width: 100%; height: 100vh;">
@@ -146,7 +149,11 @@
           <FillLayer paint={{ "fill-color": "black", "fill-opacity": 0.3 }} />
         </GeoJSON>
 
-        <NetworkLayer show={$mode.kind != "debug"} {showSeverances} {opacity} />
+        <NetworkLayer
+          show={$mode.kind != "debug" && $mode.kind != "osm-separate-sidewalks"}
+          {showSeverances}
+          {opacity}
+        />
 
         {#if $mode.kind == "route"}
           <RouteMode route_a={$mode.route_a} route_b={$mode.route_b} />
@@ -154,6 +161,8 @@
           <ScoreMode />
         {:else if $mode.kind == "debug"}
           <DebugMode {showSeverances} {opacity} />
+        {:else if $mode.kind == "osm-separate-sidewalks"}
+          <OsmSeparateSidewalksMode />
         {/if}
       {/if}
     </MapLibre>
diff --git a/web/src/NavBar.svelte b/web/src/NavBar.svelte
index e0e4549..f7d4a72 100644
--- a/web/src/NavBar.svelte
+++ b/web/src/NavBar.svelte
@@ -24,6 +24,14 @@
       >
     </li>
 
+    <li>
+      <button
+        on:click={() => ($mode = { kind: "osm-separate-sidewalks" })}
+        disabled={$mode.kind == "osm-separate-sidewalks"}
+        >Fix OSM sidewalk=separate</button
+      >
+    </li>
+
     <li>
       <button
         on:click={() => ($mode = { kind: "debug" })}
diff --git a/web/src/OsmSeparateSidewalksMode.svelte b/web/src/OsmSeparateSidewalksMode.svelte
new file mode 100644
index 0000000..d8a270d
--- /dev/null
+++ b/web/src/OsmSeparateSidewalksMode.svelte
@@ -0,0 +1,42 @@
+<script lang="ts">
+  import { GeoJSON, hoverStateFilter, LineLayer } from "svelte-maplibre";
+  import { SplitComponent } from "svelte-utils/top_bar_layout";
+  import { PropertiesTable, notNull } from "svelte-utils";
+  import { Popup } from "svelte-utils/map";
+  import { model } from "./stores";
+  import NavBar from "./NavBar.svelte";
+</script>
+
+<SplitComponent>
+  <div slot="top"><NavBar /></div>
+  <div slot="sidebar">
+    <h2>Fix OSM sidewalk=separate mode</h2>
+    <p>
+      This shows roads that seem to have a parallel sidewalk. Click one to open
+      OSM, then (if appropriate) add <b>sidewalk=separate</b> (or a <b>:left</b>
+      / <b>:right</b> variant).
+    </p>
+  </div>
+  <div slot="map">
+    <GeoJSON
+      data={JSON.parse(notNull($model).findSeparateSidewalks())}
+      generateId
+    >
+      <LineLayer
+        paint={{
+          "line-width": hoverStateFilter(5, 7),
+          "line-color": "red",
+        }}
+        manageHoverState
+        on:click={(e) =>
+          window.open(notNull(e.detail.features[0].properties).way, "_blank")}
+        hoverCursor="pointer"
+      >
+        <Popup openOn="hover" let:props>
+          <h2>Classified as {props.kind}</h2>
+          <PropertiesTable properties={props} />
+        </Popup>
+      </LineLayer>
+    </GeoJSON>
+  </div>
+</SplitComponent>
diff --git a/web/src/stores.ts b/web/src/stores.ts
index bca4e23..410408a 100644
--- a/web/src/stores.ts
+++ b/web/src/stores.ts
@@ -9,7 +9,8 @@ export type Mode =
   | { kind: "title" }
   | { kind: "score" }
   | { kind: "route"; route_a: [number, number]; route_b: [number, number] }
-  | { kind: "debug" };
+  | { kind: "debug" }
+  | { kind: "osm-separate-sidewalks" };
 export interface RouteGJ extends FeatureCollection {
   direct_length: number;
   route_length: number;