Inthispaper,wecarryoutananalysisofthestructuralpropertiesofthesolutionstothespeedgradient(SG)trafficflowmodel.Undertheconditionthattherelaxationeffectcanbeneglected,itisshownthata1-shockora1-rarefactionisassociatedwiththefirstcharacteristic,butontheotherhand,acontactdiscontinuityratherthana2-shockora2-rarefactionisassociatedwiththesecondcharacteristic.Sincetheexistenceofa2-shockor2-rarefactionviolatesthephysicalmechanismofthetrafficflow,theSGmodelismorereasonable.Iftherelaxationeffectcannotbeneglected,itissomewhatdifficulttocarryouttheanalyticalanalysisandthenumericalsimulationresultsshouldbeobtained.
