is function of shape, frontal area, and sides of vehicle; and denotes velocity. Essentially, resistance differs between a vehicle's contact point and the surface of the roadway. Metal wheels on metal rails have a significant advantage of overcoming resistance compared to rubber-tyred wheels on any road surface (railway 0.001g at and 0.024g at ; truck 0.009g at and 0.090 at ). In terms of cargo capacity combining speed and size being moved in a day: human can carry for per day, or 1 tmi/day (1.5 tkm/day) horse and wheelbarrow can carry 4 tmi/day (5.8 tkm/day) horse cart on good pavement can carry 10 tmi/day (14 tkm/day) fully utility truck can carry 20,000 tmi/day (29,000 tkm/day) long-haul train can carry 500,000 tmi/day (730,000 tkm/day) Most trains take 250–400 trucks off the road, thus making the road safer. In terms of the horsepower to weight ratio, a slow-moving barge requires , a railway and pipeline requires , and truck requires . However, at higher speeds, a railway overcomes the barge and proves most economical. As an example, a typical modern wagon can hold up to of freight on two four-wheel bogies. The track distributes the weight of the train evenly, allowing significantly greater loads per axle and wheel than in road transport, leading to less wear and tear on the permanent way. This can save energy compared with other forms of transport, such as road transport, which depends on the friction between rubber tyres and the road. Trains have a small frontal area in relation to the load they are carrying, which reduces air resistance and thus energy usage. In addition, the presence of track guiding the wheels allows for very long trains to be pulled by one or a few engines and driven by a single operator, even around curves, which allows for economies of scale in both manpower and energy use; by contrast, in road transport, more than two articulations causes fishtailing and makes the vehicle unsafe. Energy efficiency Considering only the energy spent to move the means of transport, and using the example of the urban area of Lisbon, electric trains seem to be on average 20 times more efficient than automobiles for transportation of passengers, if we consider energy spent per passenger-distance with similar occupation ratios. Considering an automobile with a consumption of around of fuel, the average car in Europe has an occupancy of around 1.2 passengers