What is the difference between the following traffic signs?

1. There is no difference between them.
2. Traffic sign 306 alerts about a pedestrian crossing ahead, while traffic sign 135 marks the place of the crossing.
3. Traffic sign 135 is placed in non-urban roads, while traffic sign 306 is placed in freeways.
4. Correct answer: Traffic sign 135 alerts about a pedestrian crossing ahead, while traffic sign 306 marks the place of the crossing.

WhySign 135 warns of a pedestrian crossing ahead, while the blue sign 306 stands right at the crossing.