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I'm using the Traffic Flow API - "Flow within Bounding Box" to get data on the speed of vehicles.

It returns an set of objects within CF (Current Flow): SSS & SS as follows:

 "CF": [
                {
                  "SSS": {
                    "SS": [
                      {
                        "LE": 10.29973,
                        "SP": 53.61,
                        "SU": 53.61,
                        "FF": 62.08,
                        "JF": 1.24036
                      },
                      {
                        "LE": 0.2004,
                        "SP": 26.98,
                        "SU": 26.98,
                        "FF": 62.21,
                        "JF": 6.80673
                      },
                      {
                        "LE": 6.38433,
                        "SP": 35.93,
                        "SU": 35.93,
                        "FF": 44.29,
                        "JF": 1.71658
                      },

So far I've found the following definitions:

  • "FF" = This indicates the speed on the segment at which vehicles should be considered to be able to travel without impediment. This speed is calculated as a percentile of observed speeds during non-rush hour.
  • "SP" = The average speed, capped by the speed limit, that current traffic is travelling.
  • "SU" = The average speed, uncapped by the speed limit, that current traffic is travelling

and

  • "FF" = The free flow speed on this stretch of road.
  • "SP" = Speed (based on UNITS) capped by speed limit
  • "SU" = Speed (based on UNITS) not capped by speed limit

The definitions of FF seem to imply that:

  1. This is the speed "they could" travel at and not currently travelling. So an estimate of maximum travel speed possible.
  2. There is some sort of average applied to "non rush hour traffic" and that it's an average of actual travelled speed during that time frame.
  3. There is some projected calculation based on historic data.

My question are:-

  • Q1) Which is the fastest speed of travel that vehicles are actually travelling at SU or FF?
  • Q2) Can I safely assume that vehicles have travelled at speeds indicated by FF during the time and date the API request was made.

Thanks in advance for any insight.

gareth4536
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  • 1

1 Answers1

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Please find inline answers to your questions-

Q1) Which is the fastest speed of travel that vehicles are actually travelling at SU or FF?

  • FF Free Flow speed is the reference speed, and will not change, so is time independent, hence SU is the actual speed. As you can see is that the JF (JamFactor) is above 0, so there is light congestion, so not driving the Free Flow speed.

Q2) Can I safely assume that vehicles have travelled at speeds indicated by FF during the time and date the API request was made.

  • If the request is made when there are only a few cars on the road, it could be possible that the SU exceeds the FF
anjuG
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  • Thank you, so it sounds like FF only for reference of what could be possible and not actual travelling speeds of vehicles. Much appreciated. – gareth4536 Nov 01 '20 at 09:15