diff --git a/gtfs/spec/en/reference.md b/gtfs/spec/en/reference.md
index b4569a54..2bad5036 100644
--- a/gtfs/spec/en/reference.md
+++ b/gtfs/spec/en/reference.md
@@ -268,6 +268,7 @@ Primary key (`trip_id`)
| `wheelchair_accessible` | Enum | Optional | Indicates wheelchair accessibility. Valid options are:
`0` or empty - No accessibility information for the trip.
`1` - Vehicle being used on this particular trip can accommodate at least one rider in a wheelchair.
`2` - No riders in wheelchairs can be accommodated on this trip. |
| `bikes_allowed` | Enum | Optional | Indicates whether bikes are allowed. Valid options are:
`0` or empty - No bike information for the trip.
`1` - Vehicle being used on this particular trip can accommodate at least one bicycle.
`2` - No bicycles are allowed on this trip. |
| `cars_allowed` | Enum | Optional | Indicates whether cars are allowed. Valid options are:
`0` or empty - No car information for the trip.
`1` - Vehicle being used on this particular trip can accommodate at least one car.
`2` - No cars are allowed on this trip. |
+| `safe_duration_factor`
and
`safe_duration_offset` | Float | **Optional** | Together, `safe_duration_factor` and `safe_duration_offset` allow an estimation of the longest amount of time a rider can expect the on-demand trip to take, for 95% of cases.
Data consumers are expected to use `safe_duration_factor` and `safe_duration_offset` to make the following calculation:
`SafeTravelDuration (seconds) = safe_duration_factor × DrivingDuration (seconds) + safe_duration_offset (seconds)`
where `DrivingDuration` is the time it would take a private car to travel the distance being calculated for the on-demand service, and `SafeTravelDuration` is the longest amount of time a rider can expect the on-demand trip to take.
This calculation should only apply to the portion of a trip that is on-demand. If a service is a deviated-fixed service, or if a rider trip includes a transfer from an on-demand to a fixed-route service, the duration of the fixed-route portion of the trip should be calculated according to the `departure_time` and `arrival_time` fields.|
#### Example: Blocks and service day