clarify meaning of travel time

There is currently no clear way to state the meaning of a travel time – particularly for elaborated data publication of route travel times there are various possibilities: a measured time of a journey just completed, a crude prediction for a journey just starting by summing current link travel times on the route, or a more sophisticated prediction e.g. using a statistical blend of profiles and current conditions, or a traffic flow model. One would expect travel time in a measured data publication to be for a journey just completed. However it is feasible that an elaborated data publication could be used for any of the above meanings – even a time for a journey just completed could be considered "calculated" because it is derived from two individual vehicle measurements. "Validity" for travel times does not really inform the distinction, since a limited future validity could be claimed even for measured travel times. ElaboratedData has a "forecast" flag which could distinguish between journey just completed and some kind of forecast for journey just starting, although this is not explicit. TravelTimeData has an enumeration travelTimeType which has four possible values but these do not clearly inform this particular distinction. We assume that the existing 4 literals are still required, but by themselves they are not very informative. While travelTimeType could be factored, an improvement involving less change is as follows: Add additional description to TravelTimeData definition: "When used within an ElaboratedData with forecast = true, the travel time is a forecast for a vehicle at the start of the specified location. When used within a MeasuredValue, or an ElaboratedData with forecast = false, the travel time is a calculation or measurement from vehicles at the end of the specified location". And add the following literals to the TravelTimeTypeEnum enumeration: "sum" - Travel time is the sum of current travel times on subsections of the specified location. "profile" – Travel time is based on past observations, without use of current measurements. "predictor" - Travel time is the output of a predictor, for example a blend of current and historical data, or a traffic flow model using current measurements. Consider from knowledge of requirements whether descriptions of the four existing literals can be refined to be more informative.