Source: https://guthlabs.ai/docs/instruments/road-trip-cost-splitter/mechanism/

instrument mechanism / verified-clean

# Road Trip Cost Splitter mechanism

The inspectable method record for Road Trip Cost Splitter, kept separate from implementation and qualification.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Model scope

No arithmetic formula is inherently incorrect, but the instrument is not fully specified for production use. The exact ambiguity requiring review is whether 'optional per-leg mileage' means mileage per passenger, mileage per leg, or passenger-by-leg mileage, and how those values are aggregated. The honest label should state that this is a user-selected cost-allocation calculator, not an objective determination of fairness; mileage weighting is specifically proportional-distance allocation.

- The equal-split formula is correct only when every listed person shares the cost equally: per_person = total_cost / num_people.

- The mileage-weighted formula is mathematically correct for allocating cost in proportion to distance ridden, but total_miles must be explicitly defined as the sum of all passengers' distances.

- The specification says 'optional per-leg mileage' but does not define whether mileage is supplied per passenger, per leg, or both, nor how a passenger's total mileage is calculated across legs.

- The specification does not define how the user selects equal splitting versus mileage-weighted splitting.

- The word 'fairly' is ambiguous: mileage weighting may not be fair for tolls, shared fixed costs, drivers, passengers joining or leaving mid-trip, or different vehicle occupancy by leg.

## Units and conventions

- total_cost is a nonnegative monetary amount in one currency, and all results use that same currency.

- num_people is a positive integer.

- For mileage weighting, every person's mileage uses the same distance unit, such as miles or kilometers; the unit cancels in the ratio only when used consistently.

- For mileage weighting, total_miles equals the sum of all listed passengers' miles and must be greater than zero.

- If costs are entered in cents rather than currency units, the output is also in cents unless an explicit conversion is applied.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "mode": "equal",
    "total_cost": 120,
    "currency": "USD",
    "num_people": 4
  },
  "expected_outputs": {
    "amount_owed_per_person": [
      30,
      30,
      30,
      30
    ]
  },
  "working": "per_person = total_cost / num_people = 120 / 4 = 30. Each of the 4 passengers owes $30."
}
````

## Failure boundary

**BREAK_CLAIMED** · SELF_FLAGGED

The equal-split formula is undefined when num_people is zero. The mileage-weighted formula also requires a nonzero total_miles; zero total mileage would create another undefined division.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/road-trip-cost-splitter/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/road-trip-cost-splitter/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/road-trip-cost-splitter/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche4/tranche4-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`