diff --git a/src/index.ts b/src/index.ts
index 3a3cb5f..8a3f4d3 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -2,6 +2,7 @@ export * from './fare-contract';
 export * from './offers/ticket-offer';
 export * from './rules';
 export * from './global-messages';
+export * from './transfer-risk';
 export * from './common/app-platform';
 export {ErrorResponse, HttpError} from './error-response';
 export {BookingAvailabilityType} from './offers/booking';
diff --git a/src/transfer-risk/__tests__/transfer-risk.test.ts b/src/transfer-risk/__tests__/transfer-risk.test.ts
new file mode 100644
index 0000000..4a525e0
--- /dev/null
+++ b/src/transfer-risk/__tests__/transfer-risk.test.ts
@@ -0,0 +1,246 @@
+import {
+  getTransferRisk,
+  getLegTransferRisk,
+  isTransitLeg,
+  UNLIKELY_TRANSFER_LIMIT_IN_SECONDS,
+  type TransferLeg,
+} from '..';
+
+const transitLeg = (overrides: Partial<TransferLeg> = {}): TransferLeg => ({
+  aimedStartTime: '2024-01-01T10:00:00.000Z',
+  expectedStartTime: '2024-01-01T10:00:00.000Z',
+  expectedEndTime: '2024-01-01T10:10:00.000Z',
+  serviceJourney: {id: 'ATB:ServiceJourney:1'},
+  ...overrides,
+});
+
+const footLeg = (overrides: Partial<TransferLeg> = {}): TransferLeg => ({
+  aimedStartTime: '2024-01-01T10:10:00.000Z',
+  expectedStartTime: '2024-01-01T10:10:00.000Z',
+  expectedEndTime: '2024-01-01T10:15:00.000Z',
+  serviceJourney: null,
+  ...overrides,
+});
+
+describe('getTransferRisk', () => {
+  it('passes when there is time to spare', () => {
+    expect(getTransferRisk(1)).toBeUndefined();
+    expect(getTransferRisk(600)).toBeUndefined();
+  });
+
+  it('treats a zero gap as uncertain', () => {
+    expect(getTransferRisk(0)).toBe('uncertain');
+  });
+
+  it('is uncertain down to the unlikely limit', () => {
+    expect(getTransferRisk(-60)).toBe('uncertain');
+    expect(getTransferRisk(UNLIKELY_TRANSFER_LIMIT_IN_SECONDS)).toBe(
+      'uncertain',
+    );
+  });
+
+  it('is unlikely past the limit', () => {
+    expect(getTransferRisk(UNLIKELY_TRANSFER_LIMIT_IN_SECONDS - 1)).toBe(
+      'unlikely',
+    );
+    expect(getTransferRisk(-600)).toBe('unlikely');
+  });
+
+  it('passes when the gap is not a finite number', () => {
+    expect(getTransferRisk(NaN)).toBeUndefined();
+  });
+});
+
+describe('isTransitLeg', () => {
+  it('distinguishes scheduled transit from walking', () => {
+    expect(isTransitLeg(transitLeg())).toBe(true);
+    expect(isTransitLeg(footLeg())).toBe(false);
+    expect(isTransitLeg(transitLeg({serviceJourney: undefined}))).toBe(false);
+  });
+});
+
+describe('getLegTransferRisk', () => {
+  it('catches a missed transfer between two transit legs', () => {
+    const legs = [
+      transitLeg({expectedEndTime: '2024-01-01T10:10:00.000Z'}),
+      transitLeg({expectedStartTime: '2024-01-01T10:09:00.000Z'}),
+    ];
+    expect(getLegTransferRisk(legs, 1)).toBe('uncertain');
+  });
+
+  it('reports unlikely once the gap is past the limit', () => {
+    const legs = [
+      transitLeg({expectedEndTime: '2024-01-01T10:10:00.000Z'}),
+      transitLeg({expectedStartTime: '2024-01-01T10:05:00.000Z'}),
+    ];
+    expect(getLegTransferRisk(legs, 1)).toBe('unlikely');
+  });
+
+  it('passes when there is time to spare', () => {
+    const legs = [
+      transitLeg({expectedEndTime: '2024-01-01T10:10:00.000Z'}),
+      transitLeg({expectedStartTime: '2024-01-01T10:15:00.000Z'}),
+    ];
+    expect(getLegTransferRisk(legs, 1)).toBeUndefined();
+  });
+
+  it('passes on the first leg, which nothing precedes', () => {
+    expect(getLegTransferRisk([transitLeg(), transitLeg()], 0)).toBeUndefined();
+  });
+
+  it('passes on an index outside the trip', () => {
+    expect(getLegTransferRisk([transitLeg()], 5)).toBeUndefined();
+  });
+
+  it('passes on a non-transit leg, so a walk carries no warning', () => {
+    const legs = [
+      transitLeg({expectedEndTime: '2024-01-01T10:10:00.000Z'}),
+      footLeg({expectedStartTime: '2024-01-01T10:09:00.000Z'}),
+    ];
+    expect(getLegTransferRisk(legs, 1)).toBeUndefined();
+  });
+
+  it('passes when there is no transit leg to have arrived from', () => {
+    const legs = [
+      footLeg({expectedEndTime: '2024-01-01T10:10:00.000Z'}),
+      transitLeg({expectedStartTime: '2024-01-01T10:09:00.000Z'}),
+    ];
+    expect(getLegTransferRisk(legs, 1)).toBeUndefined();
+  });
+
+  it('measures the gap from the end of an intervening walk', () => {
+    const legs = [
+      transitLeg({expectedEndTime: '2024-01-01T10:10:00.000Z'}),
+      footLeg({
+        expectedStartTime: '2024-01-01T10:10:00.000Z',
+        expectedEndTime: '2024-01-01T10:15:00.000Z',
+      }),
+      transitLeg({expectedStartTime: '2024-01-01T10:11:00.000Z'}),
+    ];
+    expect(getLegTransferRisk(legs, 2)).toBe('unlikely');
+  });
+
+  it('passes when the gap is unparseable rather than inventing a risk', () => {
+    const legs = [
+      transitLeg({expectedEndTime: 'not-a-date'}),
+      transitLeg({expectedStartTime: '2024-01-01T10:09:00.000Z'}),
+    ];
+    expect(getLegTransferRisk(legs, 1)).toBeUndefined();
+  });
+
+  describe('guaranteed interchange', () => {
+    it('passes on a guaranteed interchange with no stated wait limit', () => {
+      const legs = [
+        transitLeg({
+          expectedEndTime: '2024-01-01T10:10:00.000Z',
+          interchangeTo: {guaranteed: true},
+        }),
+        transitLeg({expectedStartTime: '2024-01-01T10:00:00.000Z'}),
+      ];
+      expect(getLegTransferRisk(legs, 1)).toBeUndefined();
+    });
+
+    it('warns when the interchange is explicitly not guaranteed', () => {
+      const legs = [
+        transitLeg({
+          expectedEndTime: '2024-01-01T10:10:00.000Z',
+          interchangeTo: {guaranteed: false},
+        }),
+        transitLeg({expectedStartTime: '2024-01-01T10:09:00.000Z'}),
+      ];
+      expect(getLegTransferRisk(legs, 1)).toBe('uncertain');
+    });
+
+    it('passes on a guaranteed interchange reached through a walk', () => {
+      const legs = [
+        transitLeg({
+          expectedEndTime: '2024-01-01T10:10:00.000Z',
+          interchangeTo: {guaranteed: true},
+        }),
+        footLeg({
+          expectedStartTime: '2024-01-01T10:10:00.000Z',
+          expectedEndTime: '2024-01-01T10:15:00.000Z',
+        }),
+        transitLeg({expectedStartTime: '2024-01-01T10:11:00.000Z'}),
+      ];
+      expect(getLegTransferRisk(legs, 2)).toBeUndefined();
+    });
+
+    it('passes when arrival is within the maximum wait time', () => {
+      const legs = [
+        transitLeg({
+          expectedEndTime: '2024-01-01T10:10:00.000Z',
+          interchangeTo: {guaranteed: true, maximumWaitTime: 300},
+        }),
+        transitLeg({
+          aimedStartTime: '2024-01-01T10:08:00.000Z',
+          expectedStartTime: '2024-01-01T10:08:00.000Z',
+        }),
+      ];
+      // Held until 10:08 + 5 min = 10:13, and we arrive at 10:10.
+      expect(getLegTransferRisk(legs, 1)).toBeUndefined();
+    });
+
+    it('treats arrival exactly at the deadline as caught', () => {
+      const legs = [
+        transitLeg({
+          expectedEndTime: '2024-01-01T10:13:00.000Z',
+          interchangeTo: {guaranteed: true, maximumWaitTime: 300},
+        }),
+        transitLeg({
+          aimedStartTime: '2024-01-01T10:08:00.000Z',
+          expectedStartTime: '2024-01-01T10:08:00.000Z',
+        }),
+      ];
+      expect(getLegTransferRisk(legs, 1)).toBeUndefined();
+    });
+
+    it('warns once arrival is past the maximum wait time', () => {
+      const legs = [
+        transitLeg({
+          expectedEndTime: '2024-01-01T10:20:00.000Z',
+          interchangeTo: {guaranteed: true, maximumWaitTime: 300},
+        }),
+        transitLeg({
+          aimedStartTime: '2024-01-01T10:08:00.000Z',
+          expectedStartTime: '2024-01-01T10:08:00.000Z',
+        }),
+      ];
+      // Held until 10:13, but we do not arrive until 10:20.
+      expect(getLegTransferRisk(legs, 1)).toBe('unlikely');
+    });
+
+    it('counts an intervening walk against the maximum wait time', () => {
+      const legs = [
+        transitLeg({
+          expectedEndTime: '2024-01-01T10:10:00.000Z',
+          interchangeTo: {guaranteed: true, maximumWaitTime: 120},
+        }),
+        footLeg({
+          expectedStartTime: '2024-01-01T10:10:00.000Z',
+          expectedEndTime: '2024-01-01T10:15:00.000Z',
+        }),
+        transitLeg({
+          aimedStartTime: '2024-01-01T10:11:00.000Z',
+          expectedStartTime: '2024-01-01T10:11:00.000Z',
+        }),
+      ];
+      // Held until 10:13, but the walk does not end until 10:15.
+      expect(getLegTransferRisk(legs, 2)).toBe('unlikely');
+    });
+
+    it('keeps the guarantee when the deadline is unparseable', () => {
+      const legs = [
+        transitLeg({
+          expectedEndTime: '2024-01-01T10:20:00.000Z',
+          interchangeTo: {guaranteed: true, maximumWaitTime: 300},
+        }),
+        transitLeg({
+          aimedStartTime: 'not-a-date',
+          expectedStartTime: '2024-01-01T10:08:00.000Z',
+        }),
+      ];
+      expect(getLegTransferRisk(legs, 1)).toBeUndefined();
+    });
+  });
+});
diff --git a/src/transfer-risk/index.ts b/src/transfer-risk/index.ts
new file mode 100644
index 0000000..9508c1b
--- /dev/null
+++ b/src/transfer-risk/index.ts
@@ -0,0 +1,9 @@
+export {
+  getTransferRisk,
+  getLegTransferRisk,
+  isTransitLeg,
+  UNLIKELY_TRANSFER_LIMIT_IN_SECONDS,
+} from './transfer-risk';
+// Exports both the value (TransferRisk.Unlikely) and the type.
+export {TransferRisk} from './types';
+export type {TransferLeg} from './types';
diff --git a/src/transfer-risk/transfer-risk.ts b/src/transfer-risk/transfer-risk.ts
new file mode 100644
index 0000000..0268eb5
--- /dev/null
+++ b/src/transfer-risk/transfer-risk.ts
@@ -0,0 +1,92 @@
+import type {TransferLeg} from './types';
+import {TransferRisk} from './types';
+
+/** Below this, the transfer is not one to count on. */
+export const UNLIKELY_TRANSFER_LIMIT_IN_SECONDS = -120;
+
+/**
+ * Classifies the gap between arriving and the next departure. Zero counts as
+ * uncertain; a non-finite gap yields undefined.
+ */
+export const getTransferRisk = (seconds: number): TransferRisk | undefined => {
+  if (!Number.isFinite(seconds) || seconds > 0) {
+    return undefined;
+  }
+  return seconds < UNLIKELY_TRANSFER_LIMIT_IN_SECONDS
+    ? TransferRisk.Unlikely
+    : TransferRisk.Uncertain;
+};
+
+/** Whether a leg is scheduled transit rather than walking, cycling and such. */
+export const isTransitLeg = (leg: TransferLeg): boolean =>
+  leg.serviceJourney != null;
+
+/**
+ * The risk of missing the leg at `index` — the service you are boarding, which
+ * is where the warning belongs. Arrival is the end of the leg immediately
+ * before, so an intervening walk counts.
+ *
+ * Undefined when the leg is not transit, when no transit leg precedes it, or
+ * when the transfer still holds. The transit check also keeps the warning off
+ * the leg leading *into* a walk: those gaps are commonly re-anchored to exactly
+ * zero, which would otherwise fire on every transfer.
+ */
+export const getLegTransferRisk = (
+  legs: TransferLeg[],
+  index: number,
+): TransferRisk | undefined => {
+  const boarding = legs[index];
+  const arriveAt = legs[index - 1];
+  if (!boarding || !arriveAt || !isTransitLeg(boarding)) return undefined;
+
+  const alightedFrom = previousTransitLeg(legs, index);
+  if (!alightedFrom) return undefined;
+  if (transferHolds(alightedFrom, boarding, arriveAt)) return undefined;
+
+  return getTransferRisk(
+    secondsBetween(arriveAt.expectedEndTime, boarding.expectedStartTime),
+  );
+};
+
+/**
+ * The transit leg you alight from, which carries the interchange. Walks back
+ * past non-transit legs: bus -> walk -> bus is measured on the (walk, bus)
+ * pair, but the first bus holds `interchangeTo`.
+ */
+const previousTransitLeg = (
+  legs: TransferLeg[],
+  index: number,
+): TransferLeg | undefined => {
+  for (let i = index - 1; i >= 0; i--) {
+    if (isTransitLeg(legs[i])) return legs[i];
+  }
+  return undefined;
+};
+
+/**
+ * Whether the interchange still guarantees the transfer. A guarantee lasts
+ * `maximumWaitTime` seconds past the connecting service's scheduled departure;
+ * absent, it waits indefinitely. Unparseable times keep the guarantee, so bad
+ * data suppresses a warning rather than inventing one.
+ */
+const transferHolds = (
+  alightedFrom: TransferLeg,
+  boarding: TransferLeg,
+  arriveAt: TransferLeg,
+): boolean => {
+  const interchange = alightedFrom.interchangeTo;
+  if (interchange?.guaranteed !== true) return false;
+  if (interchange.maximumWaitTime == null) return true;
+
+  const deadline =
+    toEpochMs(boarding.aimedStartTime) + interchange.maximumWaitTime * 1000;
+  const arrival = toEpochMs(arriveAt.expectedEndTime);
+  if (!Number.isFinite(deadline) || !Number.isFinite(arrival)) return true;
+
+  return arrival <= deadline;
+};
+
+const secondsBetween = (from: string, to: string): number =>
+  (toEpochMs(to) - toEpochMs(from)) / 1000;
+
+const toEpochMs = (isoDate: string): number => new Date(isoDate).getTime();
diff --git a/src/transfer-risk/types.ts b/src/transfer-risk/types.ts
new file mode 100644
index 0000000..a1abe9d
--- /dev/null
+++ b/src/transfer-risk/types.ts
@@ -0,0 +1,32 @@
+/** How risky a transfer is when there is no time to spare. */
+export const TransferRisk = {
+  Uncertain: 'uncertain',
+  Unlikely: 'unlikely',
+} as const;
+
+export type TransferRisk = (typeof TransferRisk)[keyof typeof TransferRisk];
+
+/**
+ * The fields the transfer rules read. Each product's own leg type satisfies
+ * this structurally, so no mapping is needed at the call site.
+ */
+export type TransferLeg = {
+  /** Scheduled departure. The reference point for `maximumWaitTime`. */
+  aimedStartTime: string;
+  expectedStartTime: string;
+  expectedEndTime: string;
+  /**
+   * Present on scheduled transit legs only. More reliable than mode or quay:
+   * a transfer walk between two stops has quays too.
+   */
+  serviceJourney?: {id: string} | null;
+  /**
+   * Entur's interchange to the next service. Transit legs only, and often not
+   * populated when a leg is fetched by id — preserve it across a refresh.
+   */
+  interchangeTo?: {
+    guaranteed?: boolean | null;
+    /** Seconds past its own scheduled departure the connecting service waits. */
+    maximumWaitTime?: number | null;
+  } | null;
+};