Add age-based bus fare allocation and young-person fare reform

changelog.d/1780.md

@@ -0,0 +1 @@
+- Add `bus_fare_spending` Household input variable (COICOP 7.3.2 bus & coach fares), the companion to the LCFS bus fare imputation in policyengine-uk-data, providing the passenger fare households pay (distinct from the ETB-based `bus_subsidy_spending`) as a building block for modelling bus fare reforms. Resolves #1779.

changelog.d/1781.md

@@ -0,0 +1 @@
+- Add age-based allocation of household `bus_fare_spending` to people (`person_bus_fare_spending`, via a provisional NTS-derived `gov.dft.bus.fare_allocation_weight_by_age`) and a young-person bus fare policy (`gov.dft.bus.young_person_fare.{age_limit,rate}` with `bus_fare_relief`), enabling age-targeted reforms such as free bus travel for under-22s. Inert by default; routed through `dft_subsidy_spending` so it counts as both an in-kind household benefit and government spending.

policyengine_uk/data/uprating_indices.yaml

@@ -8,6 +8,7 @@ gov.economic_assumptions.yoy_growth.obr.average_earnings:
 gov.economic_assumptions.yoy_growth.obr.consumer_price_index:
 - afcs_reported
 - alcohol_and_tobacco_consumption
+- bus_fare_spending
 - bsp_reported
 - carers_allowance_reported
 - child_benefit_reported

policyengine_uk/parameters/gov/dft/bus/fare_allocation_weight_by_age.yaml

@@ -0,0 +1,52 @@
+description: Provisional relative per-person weight, by age, for allocating household bus & coach fare spending across household members. Based on the National Travel Survey (England) bus trip age profile, adjusted with modelled concessionary (free-pass) paid-fare shares so it tracks fares paid rather than trips. Used only to apportion household bus_fare_spending to people (e.g. for age-targeted fare reforms); it is not a direct estimate of bus trips or fare incidence. Weights are relative (only ratios matter); normalised to 30-39 = 1.0.
+brackets:
+# 0-16: low paid use (child fares; some free school transport)
+- threshold:
+    2020-01-01: 0
+  amount:
+    2020-01-01: 0.5
+# 17-20: verified NTS peak (~79-90 bus trips/person/year), full fare-payers
+- threshold:
+    2020-01-01: 17
+  amount:
+    2020-01-01: 3.9
+# 21-29
+- threshold:
+    2020-01-01: 21
+  amount:
+    2020-01-01: 1.8
+# 30-39 (reference band)
+- threshold:
+    2020-01-01: 30
+  amount:
+    2020-01-01: 1.0
+# 40-49
+- threshold:
+    2020-01-01: 40
+  amount:
+    2020-01-01: 0.8
+# 50-59
+- threshold:
+    2020-01-01: 50
+  amount:
+    2020-01-01: 0.7
+# 60-69: concessionary onset (England free pass at state pension age 66)
+- threshold:
+    2020-01-01: 60
+  amount:
+    2020-01-01: 0.3
+# 70+: almost all travel free on concessionary passes
+- threshold:
+    2020-01-01: 70
+  amount:
+    2020-01-01: 0.07
+metadata:
+  amount_unit: /1
+  threshold_unit: year
+  type: single_amount
+  label: bus fare allocation weight by age
+  reference:
+    - title: NTS 2023 - bus trips by age (17-20 peak; mean ~25 trips/person/year, England)
+      href: https://www.gov.uk/government/statistics/national-travel-survey-2023/nts-2023-trips-by-purpose-age-mode-and-sex
+    - title: NTS0621 - frequency of local bus use, aged 60 and over (concessionary)
+      href: https://www.gov.uk/government/statistical-data-sets/nts06-age-gender-and-modal-breakdown

policyengine_uk/parameters/gov/dft/bus/young_person_fare/age_limit.yaml

@@ -0,0 +1,9 @@
+description: Age below which people are eligible for the young-person bus fare policy (e.g. 22 for the Scotland-style under-22 free bus travel scheme). Defaults to 0 so no one is eligible and the baseline is unaffected; a reform raises it to switch the policy on.
+values:
+  2020-01-01: 0
+metadata:
+  unit: year
+  label: young-person bus fare age limit
+  reference:
+    - title: Free bus travel for under-22s (Transport Scotland)
+      href: https://www.transport.gov.scot/concessionary-travel/young-persons-free-bus-travel-scheme/

policyengine_uk/parameters/gov/dft/bus/young_person_fare/rate.yaml

@@ -0,0 +1,6 @@
+description: Fraction of the normal bus & coach fare that eligible young people still pay under the young-person fare policy. 0 means free travel (the Scotland-style scheme); 0.5 a half-price discount; 1 no change. Combined with age_limit, which gates eligibility.
+values:
+  2020-01-01: 0
+metadata:
+  unit: /1
+  label: young-person bus fare rate paid

policyengine_uk/tests/test_bus_fare_age_allocation.py

@@ -0,0 +1,48 @@
+import pytest
+
+from policyengine_uk import Simulation
+
+# One household with a 19-year-old (fare-paying peak) and a 40-year-old, plus a
+# household bus fare total to apportion.
+SITUATION = {
+    "people": {
+        "young": {"age": {2026: 19}},
+        "adult": {"age": {2026: 40}},
+    },
+    "benunits": {"bu": {"members": ["young", "adult"]}},
+    "households": {
+        "home": {
+            "members": ["young", "adult"],
+            "bus_fare_spending": {2026: 470},
+        }
+    },
+}
+
+# Allocation weights from gov.dft.bus.fare_allocation_weight_by_age: 19 -> 3.9,
+# 40 -> 0.8.
+YOUNG_W, ADULT_W = 3.9, 0.8
+TOTAL_W = YOUNG_W + ADULT_W
+
+
+class TestBusFareAgeAllocation:
+    def test_allocation_splits_by_age_weight(self):
+        sim = Simulation(situation=SITUATION)
+        person_fare = sim.calculate("person_bus_fare_spending", 2026)
+        assert person_fare[0] == pytest.approx(470 * YOUNG_W / TOTAL_W)
+        assert person_fare[1] == pytest.approx(470 * ADULT_W / TOTAL_W)
+        # Allocation conserves the household total.
+        assert person_fare.sum() == pytest.approx(470)
+
+    def test_baseline_has_no_relief(self):
+        sim = Simulation(situation=SITUATION)
+        assert sim.calculate("bus_fare_relief", 2026)[0] == pytest.approx(0)
+
+    def test_under_22_free_fare_relieves_eligible_member(self):
+        reformed = Simulation(
+            situation=SITUATION,
+            reform={"gov.dft.bus.young_person_fare.age_limit": {"2026": 22}},
+        )
+        # Only the 19-year-old is eligible; their allocated fare is relieved in
+        # full (rate defaults to 0 -> free travel).
+        relief = reformed.calculate("bus_fare_relief", 2026)[0]
+        assert relief == pytest.approx(470 * YOUNG_W / TOTAL_W)

policyengine_uk/tests/test_road_fuel_volume_uprating.py

@@ -107,3 +107,42 @@ def weighted_litres(household, spending_variable, price_parameter, year):
         weighted_litres(household_2034, "diesel_spending", diesel_price, 2034)
         * (1 + road_fuel_volume(2035))
     )
+
+
+def test_bus_fare_spending_uses_cpi_uprating():
+    parameters = system.parameters
+    cpi = parameters.gov.economic_assumptions.yoy_growth.obr.consumer_price_index
+
+    dataset = UKSingleYearDataset(
+        person=pd.DataFrame(
+            {
+                "person_id": [1],
+                "person_benunit_id": [1],
+                "person_household_id": [1],
+                "age": [40],
+            }
+        ),
+        benunit=pd.DataFrame({"benunit_id": [1]}),
+        household=pd.DataFrame(
+            {
+                "household_id": [1],
+                "region": ["LONDON"],
+                "tenure_type": ["OWNED_OUTRIGHT"],
+                "council_tax": [1_500.0],
+                "rent": [0.0],
+                "household_weight": [1.0],
+                "bus_fare_spending": [100.0],
+            }
+        ),
+        fiscal_year=2025,
+    )
+
+    extended = extend_single_year_dataset(
+        dataset,
+        tax_benefit_system_parameters=parameters,
+        end_year=2026,
+    )
+
+    assert extended[2026].household["bus_fare_spending"].iloc[0] == pytest.approx(
+        100 * (1 + cpi(2026))
+    )

policyengine_uk/variables/gov/dft/bus_fare_relief.py

@@ -0,0 +1,24 @@
+from policyengine_uk.model_api import *
+
+
+class bus_fare_relief(Variable):
+    label = "young-person bus fare relief"
+    documentation = (
+        "Value of bus & coach fares met by government for eligible young "
+        "people under the young-person fare policy (gov.dft.bus.young_person_fare). "
+        "Treated as a transport subsidy: it counts as an in-kind household "
+        "benefit and as government spending. Zero under baseline, where the "
+        "age limit defaults to 0 (no one eligible)."
+    )
+    entity = Household
+    definition_period = YEAR
+    value_type = float
+    unit = GBP
+
+    def formula(household, period, parameters):
+        policy = parameters(period).gov.dft.bus.young_person_fare
+        age = household.members("age", period)
+        person_fare = household.members("person_bus_fare_spending", period)
+        eligible = age < policy.age_limit
+        relief = eligible * person_fare * (1 - policy.rate)
+        return household.sum(relief)

policyengine_uk/variables/gov/dft/dft_subsidy_spending.py

@@ -11,4 +11,5 @@ class dft_subsidy_spending(Variable):
     adds = [
         "rail_subsidy_spending",
         "bus_subsidy_spending",
+        "bus_fare_relief",
     ]

policyengine_uk/variables/gov/dft/household_bus_fare_weight.py

@@ -0,0 +1,18 @@
+from policyengine_uk.model_api import *
+
+
+class household_bus_fare_weight(Variable):
+    label = "household bus fare allocation weight"
+    documentation = (
+        "Sum across household members of the age-based bus fare allocation "
+        "weight, used as the denominator when apportioning household "
+        "bus_fare_spending to people."
+    )
+    entity = Household
+    definition_period = YEAR
+    value_type = float
+
+    def formula(household, period, parameters):
+        age = household.members("age", period)
+        weight = parameters(period).gov.dft.bus.fare_allocation_weight_by_age.calc(age)
+        return household.sum(weight)

policyengine_uk/variables/gov/dft/person_bus_fare_spending.py

@@ -0,0 +1,24 @@
+from policyengine_uk.model_api import *
+
+
+class person_bus_fare_spending(Variable):
+    label = "person bus and coach fare spending"
+    documentation = (
+        "Household bus_fare_spending apportioned to this person using an "
+        "age-based bus usage profile (NTS-derived, concessionary-adjusted). "
+        "This is a modelling allocation, not a direct measurement: LCFS "
+        "records bus fares only at household level. Lets age-targeted fare "
+        "reforms (e.g. free travel for under-22s) act on the relevant members."
+    )
+    entity = Person
+    definition_period = YEAR
+    value_type = float
+    unit = GBP
+
+    def formula(person, period, parameters):
+        age = person("age", period)
+        weight = parameters(period).gov.dft.bus.fare_allocation_weight_by_age.calc(age)
+        total_weight = person.household("household_bus_fare_weight", period)
+        household_fare = person.household("bus_fare_spending", period)
+        share = where(total_weight > 0, weight / total_weight, 0)
+        return household_fare * share

policyengine_uk/variables/input/consumption/bus_fare_spending.py

@@ -0,0 +1,17 @@
+from policyengine_uk.model_api import *
+
+# COICOP 7.3.2 (passenger transport by road: bus & coach fares). A
+# sub-component of transport_consumption, imputed separately from LCFS by
+# policyengine-uk-data so bus fare reforms can be modelled. Not added into the
+# `consumption` total, which already counts it via transport_consumption.
+
+
+class bus_fare_spending(Variable):
+    label = "bus and coach fare spending"
+    documentation = "Household spending on bus and coach fares (COICOP 7.3.2)."
+    entity = Household
+    definition_period = YEAR
+    value_type = float
+    unit = GBP
+    quantity_type = FLOW
+    uprating = "gov.economic_assumptions.indices.obr.consumer_price_index"