Instructions provided shows how to create a time attribute for a network dataset when a MPH and Length field exist.
Note: In this example, the Length_field must be in units of miles for this formula to return the correct time values.
[Length_field] / [MPH_field] * 60
The time is provided in minutes using this formula. It is usually necessary to adjust for another value. For example, multiply by 60 again to receive the time in seconds.