Oneway arrows are a standard feature on high-zoom maps to identify oneway streets

As an example area with oneways, https://maps.wikimedia.org/#17/49.28033/-123.12012

In Mapnik there are two strategies for oneway arrows: dash-symbolizers and SVG markers. The difference is how they handle corners. The former wraps around it, while the latter can stick out.

https://github.com/gravitystorm/openstreetmap-carto/pull/1840 provides some more information

One alternative strategy is chevrons, like used by the MapQuest app