The given information are:Traffic flow rate = 2100 pcphplWidth of each lane, w = 12 ft.Number of lanes = 12 (6 in each direction)Obstruction on right side = 7 ftPassenger-car equivalent speed, s = 70 mphNumber of ramps = 0.5/mileLet's determine the spacing between cars, d using the formula,d = s × 1/flow rate= 70 × 1/2100= 1/30 mile/carNow, determine the distance between the cars, Lc using the formula:Lc = d × 5280 = 1/30 × 5280 = 176 ft/carSince there are 12 lanes in total, the total width of all lanes together = 12 × 12 = 144 ftNow, determine the width available for traffic by subtracting the obstruction from the total width, which is, 144 - 7 = 137 ftConvert this into miles, which is 137/5280 = 0.026 miles/feetNow, let's determine the traffic density:Traffic density, k = number of cars/mile = flow rate/(lane width × available width for traffic)= 2100/(12 × 0.026) = 6597.22 cars/mileTherefore, the traffic density on this freeway section is 6597.22 cars/mile.Main Answer: Traffic density on the given freeway section is 6597.22 cars/mile. It is calculated by dividing the flow rate (2100 pcphpl) by lane width (12 ft) and available width for traffic (137 ft - 7 ft).Answer more than 100 words:Traffic density is defined as the number of cars on a given length of road or mile of a highway or freeway. It is the measure of the number of vehicles traveling on a given road, segment, or section. In the given question, the traffic density on a 12-lane urban freeway section is to be calculated. It is given that the flow rate is 2100 pcphpl.Using the formula d = s × 1/flow rate, the distance between cars is found to be 176 ft/car. The total width of the lanes is 144 ft, and the obstruction on the right side is 7 ft from the edge of the road. Therefore, the width available for traffic is 137 ft (144-7). Now, converting this to miles, the available width is 137/5280 = 0.026 miles/feet.Using the formula, k = flow rate/(lane width × available width for traffic), the traffic density is calculated to be 6597.22 cars/mile. Therefore, the traffic density on the given freeway section is 6597.22 cars/mile. Hence, the main answer is 6597.22 cars/mile, and the answer more than 100 words describes how to calculate traffic density and explains the formula used to obtain the answer.Conclusion: The traffic density is an essential parameter for transportation planning, traffic safety analysis, and highway capacity. It is the measure of the traffic load on the given length of the road. The traffic density can be used to evaluate the traffic congestion, capacity, and roadway design. It is a critical factor in determining the efficiency and safety of a roadway network.