Sentence:  Now she kept herself protectively ready to laugh again and sure enough he pointed at her with his index finger and said `` Toot '' ! once more and roared off into the fog , his foot evidently surprising him with the <WSD>suddenness</WSD> with which it pressed the accelerator , just as his hand did when he worked . 
Meaning of the word marked with <WSD> tags:  the quality of happening with headlong haste or without warning