A city plan to bar fume-belching vehicles and taxis and cars without passengers from Silom, Paholyothin and Ratchaprarop roads of Thailand at certain hours was attacked by the police yesterday. The Bangkok Metropolitan Administration's proposal was ill-conceived and might adversely affect motorists, said Pol Maj-Gen Boriboon Vuthipakdi, assistant city police chief. Implementing the plan would be difficult as there was no law to bar vehicles that emit white smoke.