23 C.F.R. § 490.505
All definitions in § 490.101 apply to this subpart. Unless otherwise specified in this subpart, the following definitions apply to this subpart:
Greenhouse gas (GHG) is any gas that absorbs infrared radiation (traps heat) in the atmosphere. Approximately 97 percent of on-road GHG emissions are carbon dioxide (CO2) from burning fossil fuel. Other transportation GHGs are methane (CH4), nitrous oxide (N2 O), and hydrofluorocarbons (HFCs).
Level of Travel Time Reliability is a comparison, expressed as a ratio, of the 80th percentile travel time of a reporting segment to the "normal" (50th percentile) travel time of a reporting segment occurring throughout a full calendar year.
Normal Travel Time (or 50th percentile travel time) is the time of travel to traverse the full extent of a reporting segment which is greater than the time for 50 percent of the travel in a calendar year to traverse the same reporting segment.
Reference year is calendar year 2022 for the purpose of the GHG measure.
Travel time cumulative probability distribution means a representation of all the travel times for a road segment during a defined reporting period (such as annually) presented in a percentile ranked order as provided in the travel time data set. The normal (50th percentile) and 80th percentile travel times used to compute the Travel Time Reliability measures may be identified by the travel time cumulative probability distribution.
23 C.F.R. §490.505