QGIS: visualisation of the directions of the roads

I was asked about the possibility to visualise directions of the roads in QGIS. Actually it is quite simple assuming that your data is topologically adequate. That means that the directions of the poly-lines representing one-way roads are the same to corresponding roads directions.

So you need a right layer with a field named like "ONE_WAY" with "yes"/"no" values. Then just check Categorised symbol in New Symbology and classify the layer using "ONE_WAY" field. Obviously you will have 2 categories: one-way and both directions.

For a one-way road create a style lake at the picture below:

Style example for a one-way road

The direction of the line in "Symbol Properties" is from left to the right, i.e. -->. In this stile I used two layers: a simple line and a marker line. An arrow marker is already present in your QGIS installation so you don't need to add anything. Such stile will represent the direction of the one-way roads. Now you only need to assign a stile for the roads wit both directions. For example the same simple line but without marker layer.