軽 - LIGHT Vehicles Travel Faster

Readings & Meanings:

かる（い）

ケイ

light (as in weight)

Common Usage:

軽い

軽重

Examples:

中身が空なのでその箱は軽かった。

鼎の軽重を問う。

彼女は男に易々と付いて行くような軽い女ではない。

Translations:

The box is light because it's empty.

To call one's ability into question. (idiom)

She is not the light, superficial kind of woman who easily goes out with men.

Radical:

車 is the radical here.

The phonetic here is a simplified version of 巠. That's 巛 (a variant of 川) running between a line on the top and 土 on the bottom. The idea is that this river runs between two points.

So, a vehicle that can run between two points has to be as light as possible (remember, it was being carried by living beings, back in the day).

References:

http://www.kanjinetworks.com/indices/radindex.html
http://www.thejapanesepage.com/node/kanji/7.htm