diff --git a/src/repositories/shuttle/eta/BaseInMemoryETARepository.ts b/src/repositories/shuttle/eta/BaseInMemoryETARepository.ts
new file mode 100644
index 0000000..1a2b7cf
--- /dev/null
+++ b/src/repositories/shuttle/eta/BaseInMemoryETARepository.ts
@@ -0,0 +1,65 @@
+import { IEta } from "../../../entities/ShuttleRepositoryEntities";
+import { ETAGetterRepository, ETARepositoryEventListener, ETARepositoryEventName } from "./ETAGetterRepository";
+import EventEmitter from "node:events";
+
+export abstract class BaseInMemoryETARepository extends EventEmitter implements ETAGetterRepository {
+  protected etas: IEta[] = [];
+
+  async getEtasForShuttleId(shuttleId: string): Promise<IEta[]> {
+    return this.etas.filter(eta => eta.shuttleId === shuttleId);
+  }
+
+  async getEtasForStopId(stopId: string): Promise<IEta[]> {
+    return this.etas.filter(eta => eta.stopId === stopId);
+  }
+
+  async getEtaForShuttleAndStopId(shuttleId: string, stopId: string): Promise<IEta | null> {
+    const eta = this.etas.find(eta => eta.stopId === stopId && eta.shuttleId === shuttleId);
+    return eta ?? null;
+  }
+
+  // EventEmitter overrides for type safety
+  override on<T extends ETARepositoryEventName>(
+    event: T,
+    listener: ETARepositoryEventListener<T>,
+  ): this;
+  override on(event: string | symbol, listener: (...args: any[]) => void): this {
+    return super.on(event, listener);
+  }
+
+  override once<T extends ETARepositoryEventName>(
+    event: T,
+    listener: ETARepositoryEventListener<T>,
+  ): this;
+  override once(event: string | symbol, listener: (...args: any[]) => void): this {
+    return super.once(event, listener);
+  }
+
+  override off<T extends ETARepositoryEventName>(
+    event: T,
+    listener: ETARepositoryEventListener<T>,
+  ): this;
+  override off(event: string | symbol, listener: (...args: any[]) => void): this {
+    return super.off(event, listener);
+  }
+
+  override addListener<T extends ETARepositoryEventName>(
+    event: T,
+    listener: ETARepositoryEventListener<T>,
+  ): this;
+  override addListener(event: string | symbol, listener: (...args: any[]) => void): this {
+    return super.addListener(event, listener);
+  }
+
+  override removeListener<T extends ETARepositoryEventName>(
+    event: T,
+    listener: ETARepositoryEventListener<T>,
+  ): this;
+  override removeListener(event: string | symbol, listener: (...args: any[]) => void): this {
+    return super.removeListener(event, listener);
+  }
+
+  override removeAllListeners(eventName?: string | symbol): this {
+    return super.removeAllListeners(eventName);
+  }
+}
diff --git a/src/repositories/shuttle/eta/InMemoryExternalSourceETARepository.ts b/src/repositories/shuttle/eta/InMemoryExternalSourceETARepository.ts
index 9e6afa4..b769c12 100644
--- a/src/repositories/shuttle/eta/InMemoryExternalSourceETARepository.ts
+++ b/src/repositories/shuttle/eta/InMemoryExternalSourceETARepository.ts
@@ -1,40 +1,13 @@
-import { EventEmitter } from "stream";
 import { IEta } from "../../../entities/ShuttleRepositoryEntities";
-import { ETARepositoryEventName, ETARepositoryEventListener } from "./ETAGetterRepository";
 import { ExternalSourceETARepository } from "./ExternalSourceETARepository";
+import { BaseInMemoryETARepository } from "./BaseInMemoryETARepository";
 
-export class InMemoryExternalSourceETARepository extends EventEmitter implements ExternalSourceETARepository {
+export class InMemoryExternalSourceETARepository extends BaseInMemoryETARepository implements ExternalSourceETARepository {
   addOrUpdateEtaFromExternalSource(eta: IEta): Promise<void> {
     throw new Error("Method not implemented.");
   }
+
   removeEtaIfExists(shuttleId: string, stopId: string): Promise<IEta | null> {
     throw new Error("Method not implemented.");
   }
-  on<T extends ETARepositoryEventName>(event: T, listener: ETARepositoryEventListener<T>): this {
-    throw new Error("Method not implemented.");
-  }
-  once<T extends ETARepositoryEventName>(event: T, listener: ETARepositoryEventListener<T>): this {
-    throw new Error("Method not implemented.");
-  }
-  off<T extends ETARepositoryEventName>(event: T, listener: ETARepositoryEventListener<T>): this {
-    throw new Error("Method not implemented.");
-  }
-  addListener<T extends ETARepositoryEventName>(event: T, listener: ETARepositoryEventListener<T>): this {
-    throw new Error("Method not implemented.");
-  }
-  removeListener<T extends ETARepositoryEventName>(event: T, listener: ETARepositoryEventListener<T>): this {
-    throw new Error("Method not implemented.");
-  }
-  getEtasForShuttleId(shuttleId: string): Promise<IEta[]> {
-    throw new Error("Method not implemented.");
-  }
-  getEtasForStopId(stopId: string): Promise<IEta[]> {
-    throw new Error("Method not implemented.");
-  }
-  getEtaForShuttleAndStopId(shuttleId: string, stopId: string): Promise<IEta | null> {
-    throw new Error("Method not implemented.");
-  }
-  removeAllListeners(eventName?: string | symbol | undefined): this {
-    throw new Error("Method not implemented.");
-  }
 } 
diff --git a/src/repositories/shuttle/eta/InMemorySelfUpdatingETARepository.ts b/src/repositories/shuttle/eta/InMemorySelfUpdatingETARepository.ts
index ce0160b..de42361 100644
--- a/src/repositories/shuttle/eta/InMemorySelfUpdatingETARepository.ts
+++ b/src/repositories/shuttle/eta/InMemorySelfUpdatingETARepository.ts
@@ -1,10 +1,8 @@
-import { IEta } from "../../../entities/ShuttleRepositoryEntities";
-import { ETARepositoryEventName, ETARepositoryEventListener } from "./ETAGetterRepository";
 import { SelfUpdatingETARepository } from "./SelfUpdatingETARepository";
 import { ShuttleGetterRepository, ShuttleTravelTimeDataIdentifier, ShuttleTravelTimeDateFilterArguments } from "../ShuttleGetterRepository";
-import { EventEmitter } from "stream";
+import { BaseInMemoryETARepository } from "./BaseInMemoryETARepository";
 
-export class InMemorySelfUpdatingETARepository extends EventEmitter implements SelfUpdatingETARepository {
+export class InMemorySelfUpdatingETARepository extends BaseInMemoryETARepository implements SelfUpdatingETARepository {
   constructor(
     readonly shuttleRepository: ShuttleGetterRepository
   ) {
@@ -14,34 +12,8 @@ export class InMemorySelfUpdatingETARepository extends EventEmitter implements S
   getAverageTravelTimeSeconds(identifier: ShuttleTravelTimeDataIdentifier, dateFilter: ShuttleTravelTimeDateFilterArguments): Promise<number | undefined> {
     throw new Error("Method not implemented.");
   }
+
   startListeningForUpdates(): void {
     throw new Error("Method not implemented.");
   }
-  on<T extends ETARepositoryEventName>(event: T, listener: ETARepositoryEventListener<T>): this {
-    throw new Error("Method not implemented.");
-  }
-  once<T extends ETARepositoryEventName>(event: T, listener: ETARepositoryEventListener<T>): this {
-    throw new Error("Method not implemented.");
-  }
-  off<T extends ETARepositoryEventName>(event: T, listener: ETARepositoryEventListener<T>): this {
-    throw new Error("Method not implemented.");
-  }
-  addListener<T extends ETARepositoryEventName>(event: T, listener: ETARepositoryEventListener<T>): this {
-    throw new Error("Method not implemented.");
-  }
-  removeListener<T extends ETARepositoryEventName>(event: T, listener: ETARepositoryEventListener<T>): this {
-    throw new Error("Method not implemented.");
-  }
-  getEtasForShuttleId(shuttleId: string): Promise<IEta[]> {
-    throw new Error("Method not implemented.");
-  }
-  getEtasForStopId(stopId: string): Promise<IEta[]> {
-    throw new Error("Method not implemented.");
-  }
-  getEtaForShuttleAndStopId(shuttleId: string, stopId: string): Promise<IEta | null> {
-    throw new Error("Method not implemented.");
-  }
-  removeAllListeners(eventName?: string | symbol | undefined): this {
-    throw new Error("Method not implemented.");
-  }
 }