Traffic modeling terms, abbreviations and acronyms
Here are the main technical terms used in macroscopic traffic modelling and Q-Traf. Some abbreviations or acronyms are used in the files names and fields names of the QGIS layers generated by Q-Traf.
| Time period | APH, PPH, AAWT, AADT | Macroscopic traffic models work over a fixed time period, generally “1 hour”. Q-Traf allows to choose between 2 time periods for all the analysis : – 1 hour – 24 hours (the time period is chosen in the Tab “Scenarios”) Note that the acronyms (APH / PPH, and AAWT / AADT, see below) are only used in file names (matrix and result files), to distinguish them (and not in calculations). All the data are and must be relative to the chosen time period (1 or 24 hours) : – Trips matrix : Number of vehicles (trips) – Road capacity (in the Tab “Links categories”) – Curves Time(flow) (in the Tab “Curves”) Hence, in the results, “volume”, “flow” and “flow rate” (whose definition is the flow divided by the time period) are all equal and relative to the time period (If “flow” is the quantity of vehicles during the period P, the “flow rate” is the quantity during the same period). It is possible to “trick” the software and work with other time periods, for example “3 hours”. One just needs to : – Fill in the trips matrices with the values for 3 hours – set links capacities for 3 hours (in the column “hourly capacity” or “Daily capacity” of the tab “links categories”) – adjust the “time(flow)” curves (tab “curves”) if necessary |
| Traffic volume | Vol, Flow | Quantity of traffic. In transportation engineering, there are 2 units for traffic volume or quantities : – Number of vehicles (or vehicles count) – PCE / PCU |
| Number of vehicles | Veh, All_veh | One of the 2 units of measurement of road traffic The number of vehicles is simply the number of vehicles : Cars + trucks. Eventually + Motorcycles + Buses + … |
| Passenger car equivalent (PCE) or passenger car unit (PCU) | PCE / PCU | The other unit of measurement of road traffic, and the only -regular- one to assess a road “capacity”. One can understand that “1000 heavy trucks per hour” and “1000 cars per hour” does not represent the same “load”, or the same “traffic flow rate”. But they are both “1000 vehicles per hour”. So how to do ? By simply using the relative inconvenience or trouble of each vehicle category, or “personal car equivalence factor”. For instance : 1 car : 1.0 pce (by definition) 1 motorcycle : 0.4 pce 1 truck or bus : 2.0 pce Hence “1000 trucks /h” is “2000 PCE/h”, where “1000 cars /h” is only “1000 PCE/h” A mixed traffic can also be easily quantified : “(1000 trucks + 1000 cars) /h” is “3000 PCE/h”. |
| (traffic flow) rate | Q | The (traffic flow) rate is a volume divided by a period of time. “Volume vs rate” : In Q-Traf, “Volume” and “Rate” are not distinguished because they are identical (see above). In the geojson result files, “rate” is generally noted “Q” or “PCE”. |
| vehicles vs PCE/PCU units | In Q-Traf and the geojson (QGIS layers) result files : “vehicles” and “PCE/PCU” are always distinguished : – “PCE” is always noted “PCE”, and – the “”volume” or “number of vehicles” is noted “vol”, “all_veh” or “flow”. | |
| Road capacity | C | The road capacity is the maximum sustainable traffic flow rate of a road. The unit of road capacity is “PCE (PCU) /hour” For convenience, in Q-Traf, the flow rate can be expressed for “24 hours” (in the Tab “Links Categories”) for “24 hours” analysis, to allow to draw LOS maps for daily traffic. |
| Level of Service | LOS | It is the ratio : “traffic flow rate” / “capacity”, with traffic flow rate and capacity in “PCE (PCU) per hour”. For convenience for the 24h time period, QGIS and the results of Q-Traf allows to draw a LOS map for the “24 hour” period, and for a traffic flow in “number of vehicles”. |
| Degree of saturation | DOS | see LOS |
| Annual Average Daily Traffic | AADT | Average number of vehicles per day over one full year expressed in “vehicles/24h” |
| Average weekday Traffic | AAWT | Average number of vehicles per weekday day expressed in “vehicles/24h” |
| Origin-Destination | OD | Each trip starts from an origin and ends at a destination |
| Trips | Trips are stored in the matrix files, with the origins listed in the first column and the destinations listed in the first row. For the moment, in Q-Traf, trips must be in “number of vehicles” (“hourly number of vehicles” for hourly analysis, and “daily number of vehicles” for daily analysis) (but one can apply a coefficient on each matrix in the UI) | |
| Free Flow Speed | The desired speed on the road, when the vehicle density is near zero — without interaction between vehicle | |
| Tree, tree plotting, arborescence | tree, arbo | All the paths followed by trips from a selected link, or a selected zone |
| Free assignment | Free assignments assign all trips via the minimum time-paths (or minimum generalized cost-paths) between zones, based on “free flow” speeds (First/single iteration. See Tabs “Calculations”) | |
| Restrained assignment | Trips still follow minimum paths, but path times are increased as the ratio “assigned volume / link capacity” increases in the assignment of trips. Original minimum paths increase In time/cost until another path may become faster, the net effect being more evenly distributed travel and more realistic simulation of traffic flows. |
