No one abides by them anyway. People don't care. If the speed limit is 35. they go 60. If it's 80, they go 95 or 100. Aren't we just wasting metal by erecting these signs all over our roads?
The police are powerless to enforce these limits anyway. For one thing, it's impossible to catch all people speeding at one time. For another thing, people who receive speeding tickets seldom even pay them - they either ignore the notice (especially if their vehicle is registered out of state), or they dispute it in court and have the fine thrown out.
Other methods to enforce speed limits (such as cameras) don't succeed either, because people whine about these and they eventually are scrapped due to public pressure. Phoenix tried this and they're on the verge of doing away with their system of speed cameras because the public's opposition has been too overwhelming.

