Laboratory 4 – Safe Following Distance and Traffic Density

1. Learning Objectives/Outcomes Be able to apply the speed and distance relationship to assess car-following safety. Be able to estimate the space mean speed of vehicles based on measured densities and

an assumed relationship between speed and density. Be able to relate the relationship between microscopic traffic parameters and

macroscopic traffic parameters.

2. Textbook References Section 5.2 – Traffic stream parameters Section 5.3 – Basic traffic stream models

3. BackgroundHow do drivers choose their speeds when driving on a highway or freeway? And, how do they decide how close to follow a vehicle (or, what is the safe following distance?)? There are a number of factors that contribute to the individual choices that drivers make in both selecting a speed and selecting a safe following distance. Some of these factors include:

1. Pavement conditions: Most drivers will drive more conservatively if it is rainy and the pavement conditions are wet or if the pavement is covered with ice or snow.

2. Time of day: Drivers tend to drive more conservatively at night when visibility is lessened, than they do during the day, when it is bright and sunny.

3. Roadway geometry: Drivers tend to drive more slowly on curved sections than on tangent sections.

4. The driver ahead: The behavior of the driver ahead in the traffic stream will influence the speed that you travel and the distance behind (spacing) that you decide to follow.

5. Traffic density: Drivers tend to reduce their speeds as the density of traffic increases.

Notice that the last two points relate to traffic conditions, one of which is on an individual driver level and the other dealing with traffic conditions in the aggregate. We sometimes call the first perspective “microscopic” and the second perspective “macroscopic”. In this lab we will learn more about how drivers behave in the field from both perspectives.

You will answer the following questions as part of this lab:1. Do vehicles follow at safe distances? You will measure following distances of drivers on

the Moscow-Pullman Highway and relate these findings to microscopic issues relating to safe following distances.

2. How can you estimate the space mean speed of vehicles if you know the average density or spacing of traffic? You will measure traffic densities along State Route 118, a freeway in Los Angeles, and use a macroscopic model to estimate average vehicle speeds on the freeway.

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3. How can you relate the microscopic view of traffic (looking at individual vehicles) and the macroscopic view of traffic (looking at average vehicle flows)?

4. AssignmentQuestion 1: Do Vehicles Follow At Safe Distances?

Step 1: Open Google Earth and fly to the Moscow-Pullman Highway. Select the “Add Placemark” tool. Set the latitude (46°43'56.28"N) and longitude (117° 5'7.08"W) of a new placemark. Set the range to 228 meters and the tilt to zero degrees. This location is the western boundary of the area from which you will collect data. Another description of this western boundary is that it is the boundary between the

lower resolution pictures in Pullman and the higher resolution pictures in Moscow. The highway in this view is the Moscow-Pullman Highway.

Step 2: Traveling from this western point eastward, identify all pairs of vehicles that are traveling relatively close together.

“Close together” means that the following vehicle in the pair seems to be influenced by the leading vehicle. This is going to be a judgment call on your part.

For each vehicle pair that you identify, use the “Ruler” tool to measure the distance from the rear of the leading vehicle to the front of the following vehicle (space gap). Specify the measurement distance in feet.

Record each measurement in a spreadsheet file. There are 15 such vehicle pairs from the western boundary described above and the intersection of the highway with Warbonnet Drive in Moscow.

Prepare a column chart showing each measurement in order from high to low.

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Step 3: Determining safe following distance. Since you have only the information that you can gather from these aerial photographs,

you can’t precisely tell if the vehicles are following at safe distances or not. However, you can make some judgments about the spacings that you measure.

There are several “rules of thumb” regarding following distance. Wikipedia describes the two-second rule, which is often used to give drivers a way of judging safe following distances (http://en.wikipedia.org/wiki/Two-second_rule_(road)):

“The two-second rule is a rule of thumb by which a driver may maintain a safe following distance at any speed. The rule is that a driver should ideally stay at least two seconds behind any vehicle that is directly in front of the driver's vehicle. It is intended for automobiles, although its general principle applies to other types of vehicles. The two-second rule is useful as it works at any speed. It is equivalent to one vehicle-length for every 8 km/h (5 mph) of the current speed, but drivers can find it difficult to estimate the correct distance from the car in front, let alone to remember the stopping distances that are required for a given speed, or to compute the linear equation on the fly. The two-second rule gets around these problems, and provides a simple and common-sense way of improving road safety.”

Using the two-second rule, and assuming the vehicles are traveling at 55 miles per hour, determine the proportion of your sample of vehicles that is following safely and the proportion that is not.

A second test is to compute the following distance required that provides adequate distance for the following vehicle to stop if the leading vehicle decelerates to a stop, if both vehicles stop using normal deceleration rates of 11.2 feet per second per second. If both vehicles are traveling at 55 miles per hour, compute this safe following distance.

Using the results from this second test, again determine the proportion of your sample of vehicles that is following safely and the proportion that is not.

Question 2: How can you estimate the average speed of vehicles if you know the average density or spacing of traffic?

Step 1: Open Google Earth and fly to the State Route 118 in Los Angeles. Select the “Add Placemark” tool. Set the latitude (34°16'20.28"N) and longitude (118°33'21.24"W) of a new placemark. Set the range to 228 meters and the tilt to zero degrees. Locate the freeway overpass to the left of the map field of view.

Step 2: Estimate the jam density. Use the on ramp (that is being metered) in the lower left of your field of view to

calculate the jam density. Measure the distance from the first vehicle stopped on the

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ramp in the right lane to the last vehicle in the queue on the ramp. Compute the number of vehicles per mile; this is an estimate of the jam density.

Step 3: Estimate the density for 12 freeway segments. For a distance of about 2400 feet from this point on the freeway traveling to the right

(east), draw lines that are about 200 feet in length and cover all lanes serving the direction of traffic moving from left to right. For each line, record the number of vehicles in this length of freeway and the number of lanes in the freeway traveling in that direction. This will give you about 12 such measurements.

Compute the density of traffic for each line, in terms of vehicles per mile per lane.

Step 4: Estimate the average speed in each segment. Using the macroscopic model that related space mean speed to measured density, jam

density, and free flow speed (equation 5-15), estimate the space mean speed of vehicles for each density measurement. Assume that the free flow speed is 75 miles per hour.

Review the range of vehicle speeds for the 12 segments. Are these speeds reasonable for a freeway? Why or why not?

Step 5: Use the model to determine other traffic flow characteristics of this segment of freeway. Based on the model that you have developed, what is your estimate of the capacity of

this section of freeway? How sensitive is your estimate of capacity to your assumption of free flow speed. How does the discussion on page 146 of your textbook on the three relationships

between speed and density affect your conclusions about the accuracy of your speed estimates from step 4?

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Question 3: How can you relate the microscopic view of traffic (looking at individual vehicles) and the macroscopic view of traffic (looking at average vehicle flows)?

Step 1: Microscopic and macroscopic views of density. Equation 5.13 in your textbook relates density (k) to spacing (s). What have you learned

in answering the first two questions that relates the microscopic view of traffic flow (as described by spacing) with the macroscopic view of traffic flow (as described by density)?

How is safe following distance related to traffic density? How is the distance that you would follow a vehicle affected by the density of traffic?

5. ReportPrepare a two page report in which you summarize your answers to the questions that you considered during this laboratory. The report should include the following sections:

1. Laboratory title.2. Names of the authors of the report.3. Date that the report was completed.4. Answers to the three questions that were posed during the lab.5. One paragraph describing the main points that you learned from this laboratory.

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