Turn penalties and intersection modeling
Q-traf does not allow to set turn penalties for an intersection defined as a single point / link ends (Q-Traf algorithm can’t manage “turn penalty” tables without requiring much more RAM). Penalties can only be defined on links (Tabs “Links categories” and “Curves” in Q-Traf). To set turning penalties, it is necessary to draw each turn as a link (in the QGIS links layer) :
| Base intersection | 4-links intersection | 12-links intersection Each turn movement is a link |
 |  |   |
| => No turn penalty can be specified in Q-Traf | Each turn penalty can be specified through the link category Create and use specific “link category” for the turning movement links. (beware that there are only 50 categories allowed) |
This representation has its advantages and disadvantages :
| Drawbacks | Advantages | |
| “Turn penalty” on links | Intersections modeling is more tedious. But it is not necessary to code all intersections in detail, and the intersection pattern is always the same. | Turn penalties are not stored in a separate, linked and error-prone table, but are clearly visible in the single “network” layer (and they can be drawn with a specific color). |
How to model RHT and LHT ? What ‘s the difference ?
There are only 2 differences between “right-hand traffic” or “left-hand traffic” modeling :
| The road network / links layer are drawn differently |  |
| Drawing link flows/data in QGIS requires an opposite offset : – positive offset for RHT – negative offset for LHT |   |
=> There is no difference in the Q-Traf parameters
How to prevent unwanted U-turn ?
| U-turns are possible as soon as 2 links are connected. |  |
| To avoid it : Remove unnecessarily short links/unnecessary crossroads, |  |
| or Separate the links in each direction (at least by a few meters as a precaution, since the coordinates are rounded to the meter) |  |
How to model a roundabout ?
| Avoid links too short for entering and exiting roundabouts ; In the event of congestion on the ring, some of the traffic could be diverted |   |
| Use long enough entry and exit links |  |