If you have any opinions about devices for slowing cars down (humps, narrowings, etc.), please write them down.
Literal
Car [genitive-の] speed [object-を] drop device (humps [and-や] narrowings) [about-について], opinion [subject-が] [if-あったら] please-write.
A form-feedback prompt. 狭さく (きょうさく) is the architectural/civil-engineering term for 'narrowing' — a road design feature that narrows a street to slow traffic. ハンプ (hump) is a loanword for 'speed hump/bump.' Both are traffic-calming infrastructure. The phrase [verb stem]方 here turns into the pre-noun modifier 落とす ('drop, lower') modifying 装置 ('device'): 速度を落とす装置 = 'a device that lowers speed.' The frame ~があったら~てください ('if there is X, please do Y') is the standard polite invitation for optional feedback. 意見 ('opinion, view') is the object of writing.