Snow tends to accumulate better on grass surfaces than concrete surfaces for several reasons.
(1) A road surface is connected directly to the earth's surface. Vegetation and grass is more exposed to the cold air. It takes time for the soil temperatures do adjust to colder air temperatures. Warmer soils will continuously conduct heat upward. A road will continue to be warmed by the warmer soil below. Bridges and overpasses will freeze before roads because there is cold air on both the top and bottom portion of a bridge or overpass.
(2) Roads are a good absorber of radiation. Roads take longer to cool off than vegetation.
(3) The shadow effect in grass and vegetation allows for less melting of snow. A road surface is completely exposed to the sun's radiation. Even on a cloudy day, radiation will still make it to the earth's surface. Places shaded from solar energy (within grass blades, behind bushes) keep radiation from sublimating the snow.
(4) Vehicles warm roads. A high volume of traffic on a roadway will effectively melt snow. This melting occurs from two sources, friction and vehicle exhaust. The friction between car tires and the road/snow can warm the temperature up enough to melt the snow / ice.
(5) The smooth surface of a road makes it difficult for snow to accumulate on a road surface, especially when wind speeds are high. The grass blades and bushes slow the wind and provide traps for the snow to settle.
(6) Snow that falls into water will melt more quickly than snow falling on a dry surface. Vegetated regions are much better at absorbing excess water than roadways. When ponding occurs on road, snowflakes will melt quickly as they fall into the water.
(7) When roads are salted or other melting agents are applied to roads, the accumulation will be reduced or eliminated. Salt from one winter storm can linger on the roads and help melt winter precipitation that occurs in a future storm.