Source: US Census Bureau. American Community Survey Reports, Commuting in the United States: 2009. & Commodity Flow Survey data. For shipments less than 50 miles on all modes.
In urban areas, trucks and cars share the roads, implying that when there is congestion, the road capacity used by one mode is at the expense of the other. Although cars and trucks have different trip distribution patterns and are related to different economic and social activities, they both have peak periods of activity between 8 AM and noon on a typical weekday. Most trucks make their deliveries immediately after the morning peak hour when businesses are open and able to receive shipments.
Morning deliveries are often preferred since stores are less busy with more employees available for unloading, unpacking, and stacking. Goods are thus available for the more active afternoon and evening retail activities. This pattern compounds congestion problems during the morning and afternoon since they correspond to periods of high activity for both car and truck movements.