diff --git a/worker/src/jobs/build-valhalla.ts b/worker/src/jobs/build-valhalla.ts
index 2005662..581e23d 100644
--- a/worker/src/jobs/build-valhalla.ts
+++ b/worker/src/jobs/build-valhalla.ts
@@ -1,3 +1,4 @@
+import { cpus } from "os";
 import type { Job } from "bullmq";
 import { execSync, spawn } from "child_process";
 import { existsSync, mkdirSync, readFileSync, readdirSync, rmSync, statSync, unlinkSync, writeFileSync } from "fs";
@@ -61,6 +62,13 @@ const INCLUDE_TRANSIT = (process.env.VALHALLA_INCLUDE_TRANSIT ?? "").toLowerCase
 
 const ROAD_BBOX_BUFFER = 0.2;
 
+/**
+ * Cap Valhalla worker thread count. Defaults to CPU count which can be 256+
+ * on large hosts — causes OOM for cities with large transit datasets (Berlin etc.).
+ * Override with VALHALLA_CONCURRENCY env var.
+ */
+const VALHALLA_CONCURRENCY = parseInt(process.env.VALHALLA_CONCURRENCY ?? String(Math.max(1, Math.floor(cpus().length / 2))), 10);
+
 const ROUTING_MANIFEST = `${VALHALLA_DATA_DIR}/routing-sources.json`;
 
 function readManifest(): Record<string, string> {
@@ -80,9 +88,9 @@ function runProcess(cmd: string, args: string[]): Promise<void> {
     console.log(`[build-valhalla] Running: ${cmd} ${args.join(" ")}`);
     const child = spawn(cmd, args, { stdio: "inherit" });
     child.on("error", reject);
-    child.on("exit", (code) => {
+    child.on("exit", (code, signal) => {
       if (code === 0) resolve();
-      else reject(new Error(`${cmd} exited with code ${code}`));
+      else reject(new Error(`${cmd} exited with code ${code}${signal ? ` (signal: ${signal})` : ""}`));
     });
   });
 }
@@ -148,6 +156,7 @@ function writeCityTransitIngestConfig(citySlug: string): string {
       admin: ADMINS_SQLITE,
       transit_dir: cityTransitCacheDir(citySlug),
       transit_feeds_dir: cityGtfsFeedsParentDir(citySlug),
+      concurrency: VALHALLA_CONCURRENCY,
     },
     service_limits: { isochrone: { max_contours: 5 } },
   } as JsonObject);
@@ -170,6 +179,7 @@ function writeCityConfig(citySlug: string, cityTileDir: string): string {
       tile_extract: `${cityTileDir}.tar`,
       timezone: TIMEZONE_SQLITE,
       admin: ADMINS_SQLITE,
+      concurrency: VALHALLA_CONCURRENCY,
       ...(INCLUDE_TRANSIT ? {
         transit_dir: cityTransitCacheDir(citySlug),
         transit_feeds_dir: cityGtfsFeedsParentDir(citySlug),