RELATED RESOUCES:
Observing vehicle speeds is one way traffic engineers measure travel safety on roadways. High speeds carry a high risk, whereas low speeds are relatively safe. In addition, observed speeds are used in capacity analysis, crash analysis, geometric studies, determining the need for pedestrian crossings, and in before-and-after studies to assess the effectiveness of roadway modifications. Also, speed observations are compared to the posted speed limit to help determine whether the roadway is in need of greater law enforcement, realignment, or reconstruction. With so many important decisions being based on observed speeds, it is imperative that such speed data be collected correctly and presented effectively.
The equipment list for data collection procedure is: a stopwatch (cell phone), measuring tape/wheel, and note taking materials.
Visit the site, and choose a location and time to take observations
For this lab, the student groups will gather data during the off-peak period during daylight hours on a Friday.
Mark the speed trap
The student groups will measure and mark a section of the road long enough to start a stopwatch when a vehicle enters the trap and stop it when the vehicle exists the trap. The recorded time is later used to compute vehicle speed. The trap’s length should be governed by the anticipated speeds of the observed traffic. Minimum suggested lengths are shown below:
Recommended Trap Length
If Speed \(< 25 mph\) then min. length : \(88 feet\)
If Speed \(25 - 40 mph\) then min. length : \(176 feet\)
If Speed \(> 40 mph\) then min. length : \(264 feet\)
Pavements marking should be easily visible to those timing the vehicles. Markings should be placed outside the traveled way, preferably on sidewalks or curbs. Physical features such as utility poles, tress, and signposts can be used as the trap markers.
Collect the data, and record it on the Spot Speed Study form
The student groups must use the attached Spot Speed Study Data Collection Form to record speed data. All of the information at the top of the form must be completed before data collection can proceed. When a vehicle enters the speed trap, and the watch is started; when the vehicle exists the trap the watch is stopped, and the time is recorded. The student groups should then repeat the process until at least 100 observations are obtained.
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. The student
groups should compute one or two speeds from the observed times to see
if they are reasonable.
\(V = d / 1.47t\)
where \(V\) is the speed in miles per hour, \(d\) is the length of the trap in feet, and \(t\) is the recorded time in seconds.
Construct speed groups
Class demo
Plot histogram
Class demo
Compute frequency and cumulative frequency
Class demo
Plot cumulative frequency curve
Class demo
Determine
Class demo
Pace: the range of speed—usually taken at 10 mph intervals—that has the greatest number of observations.
The last step is to draw conclusion based on the observed speed data.