The sophistication and extent of a ramp metering system is based on the amount of improvement desired, existing traffic conditions, installation costs, and the continuing resource requirements that are necessary to operate and maintain the system effectively. The simplest form of control is a fixed time operation. It performs the basic functions of breaking up platoons into single-vehicle entries and setting an upper limit on the flow rates that enter the freeway. Presence and passage detectors may be installed on the ramp to actuate and terminate the metering cycles, but the metering rate is based on average traffic conditions at a particular ramp at a particular time. This type of operation provides the benefits associated with accident reductions, but is not as effective in regulating freeway volumes because there is no input about mainline traffic. Pre-timed control can be implemented on any number of ramps, and is often implemented as an initial operating strategy until individual ramps can be incorporated into a traffic responsive system.
THE sophisticAtiOn And eXTenT oF a ramp meTErInG sYstEm is BaseD on the Amount of impROvemenT DeSirED, existinG Traffic COndITIONs, iNsTaLlAtIon COsts, and tHE ConTiNuing REsOurce REquIreMEnTS thAt ArE NEcessaRY To opErate aNd mAIntaIn thE sysTem EFfeCtiveLY. ThE SImPlest foRm Of cONtrOL IS a fIxED tIMe opErAtion. it PERFormS ThE Basic functions of breaking Up platOons iNto singlE-veHIcle entRiES and sETtiNg aN uppEr Limit oN The fLow rAteS thaT enTer tHe FrEeWAy. PResence aNd paSSagE DetectoRS may be installEd on THE RAmp To actuaTE and TeRminaTe thE metEriNG cycles, BUt tHe meterIng rAtE Is based oN avErage traFfic conditIONs At a pArtICulAr ramp At A particular tiMe. ThiS Type of opEraTion PRovidEs the beNeFitS aSsOciatEd WiTh accIDeNt reDuctionS, but is Not as efFectIvE In reguLaTiNG freewAy volumEs bECAuSe there is no inPUt abOut mainliNE trAffic. pre-tiMed conTroL can be iMpLeMENtEd oN any nuMbEr Of ramps, and is OftEn iMPleMEnTEd as an INItiAL OpERatINg StrAteGy untIl iNdIviduaL raMpS CaN be INcoRpOrATEd inTo A traffiC rEsPonsiVe sYsTeM.