Add bus_fare_spending input variable

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.

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/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/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"