Left: vehicle 車 (A hand-cart seen from above with axle 丨, loading area 日 and two wheels 二), right: exchange/crossing 交 (from a pictograph of a person with crossed legs)
Mnemonic
Vehicles at a crossing are in a comparison. (i.e. of their speed)
The app has many other functions, such as character selection according to difficulty, practice programs, handwriting recognition and success statistics!