Denpa Teki na Kanojo

[Ryllharu]

While that may be the impression it gives off, you can't forget that Ame (or her friends) are by no means "normal" people. Despite being an otaku, Ame is very athletic, and her friends are practically ninja. She wasn't all that far from the scene, so she might have just heard the faint sound of the truck crashing and gotten a very bad feeling, and rushed back to make sure he was alright.

You can tell she was close by because when the sun is that low in the sky, it does not take very long to set completely, and was still up. She was a five minute run away at the most.