RELATED RESOUCES:
Any traffic Study requires accurate traffic counts. Determining how many vehicles use a section of roadway or an intersection is necessary for analyzing and timing signals, determining capacity, and estimating the level of service that will be needed. Accurate counts of current traffic flow provide a departure point for estimates of future traffic volume. Moreover, the effectiveness of various traffic control measures, geometric modifications, or maintenance practices is governed by traffic volume data. The turning movement count at controlled intersections (those containing signs or signals) is another way to determine traffic volume.
The equipment list for data collection procedure is a stopwatch (cell phone) and note-taking materials.
As described the student groups will be measuring departure volumes. Departure volume denotes vehicles which are not counted until they have entered the intersection and executed their turning movements. This method serves most traffic engineering needs except when observing a signal at which the demand volume exceeds the capacity. Typically, the tuning movement counts (TMC) should start at least one hour before the projected peak hour and continue for one hour after the projected peak hour.
Collect and record the data Data is observed in fifteen minute increments throughout the count periods—usually three hours. Before starting, the times marking the beginning of each fifteen-minute observation period are recorded. Then the watch is started, and the volume count begins. During each time period, the approach that each vehicle used to enter the intersection is recorded as well as whether it turned left, right, or continued straight when exiting the intersection. The Turning Movement Count Data Collection Form has a column for each of the twelve possible movements that can occur at the intersection. At the end of each fifteen-minute period, the observer continues counting but now records the data on the next row, and so on, until the counting period is complete.
Check your work before leaving the field Before leaving the field, the data collection form should be checked to see that all field data have been collected correctly. Any unusual observations should be listed as comments.
Check the data Even though the data is checked for completeness prior to leaving the field, it is wise to check each Field Data Sheet again prior to reduction and analysis.
Summarize field sheets Class demo
Identify the peak hour, and compute the peak-hour factor The peak hour and the peak-hour factor are computed from the entries on the summary sheet. The peak hour is determined by identifying the four consecutive fifteen-minute (1-hour) periods having the highest volume. The peak-hour factor (PHF) is a valuable indicator of flow variability within the peak hour. It is computed by dividing the peak-hour volume by the Peak Rate of Flow. The Peak Rate of Flow is four times the peak fifteen-minute flow within the peak hour. PHF’s can range from a low of 0.25 to a high of 1.00. PHF of 0.25 indicates all of the peak hour traffic occurs during the one fifteen-minute interval whereas PHF of 1.00 indicates the peak hour traffic is spread out evenly throughout the hour.
Note: Computation of the peak-hour factor is not limited to the intersection as a whole. It can be computed for each approach and for each lane or lane groups as well.
Class demo
Class demo
Use the HCS7 TWSC module to create report Class demo
Interpret the data and draw conclusions Interpretation of volume data depends on the use for which the count was conducted. Among the questions to be addressed are: • Is the traffic evenly distributed among all the approaches to the intersection? • Does the peak-hour factor indicate that a surge of traffic is arriving at the intersection during particular time frame, or is the traffic spread out evenly over the peak hour? • Are there heavy turning volumes? • What percentage of the approach volume is heavy vehicles?
The last step is to draw conclusion based on the vehicle counts and the report produced from HCS7.
Note: While some of these questions may lead to useful conclusions, volume data alone cannot answer all the questions that may arise about an intersection. Additional information such as accident history, signal timing, pedestrian flow, and volume distribution throughout the remainder of the day may be needed to complete the traffic engineering analysis.