Chapter 12 VEHICLE SPOT SPEED STUDY

Transcription

1 Chapter 12 VEHICLE SPOT SPEED STUDY 12.1 PURPOSE (1) The Vehicle Spot Speed Study is designed to measure the speed characteristics at a specified location under the traffic and environmental conditions prevailing at the time of the study. Spot speed data are used in many traffic engineering activities such as determining traffic signal timing, roadway capacity, evaluating the effectiveness of improvements, and installing speed zones. (2) The location, time and conditions of the study shall be dictated by its objective and scope. If approach speeds to an intersection are needed, the measurements should be taken upstream of the intersection prior to vehicle deceleration for a possible stop at the intersection. If the study requires free-flow speeds, the measurements should be taken during off-peak time periods. The same logic should be followed for measurements needed during nighttime conditions, wet pavement, etc. (3) There are two commonly used approaches to collect vehicle speeds at spot locations: individual vehicle selection method and all-sampling vehicle method. The individual vehicle selection method entails using a manual speed measurement technique and is generally used for short-term speed measurements. The all-sampling method uses automated in-road or roadside measurement equipment (e.g., pneumatic tubes, standard induction loops, point loops, etc.) and is appropriate to use for system performance monitoring system. This chapter focuses on the individual vehicle selection method. For more information on the all-sampling method refer to the ITE Manual of Transportation Engineering Studies, 2nd Edition, Chapter 5.3. page 86. (4) Vehicle speed is determined using the direct measurement technique with radar, laser or infrared technologies which generally operate on the Doppler Principle. The positioning of the radar or laser unit should be determined considering the following: (a) The capabilities of the unit manufacturer s specifications and instructions shall be followed for the units set up and operation Vehicle Spot Speed Study 12-1

2 (b) Minimize the angle of incidence to maintain the cosine error below 2 mph it is recommended to maintain an angle of incidence of less than 15 degrees between the radar beam and the direction of travel target vehicle. (c) Conceal the unit from the view of the motorists this measure will prevent motorist distraction and reaction. (5) For a graphical example of potential positions of the radar unit review the ITE Manual of Transportation Engineering Studies, 2nd Edition, Exhibit VEHICLE SPOT SPEED STUDY FORM (1) Form No is designed to allow for several options in the collection of speed data. Data can be collected by direction, Option 1, in which case both sides (i.e., left and right of the column showing the speed ranges) of the form are used, or as a cumulative for both directions, in which case either side of the form can data by direction. This data can then be used to represent data for both directions. Using data, it is possible to calculate the 85 th percentile speed and 10 mph pace for each direction of travel. An example of this option is shown in Figure The observer enters a number 1 in a data block under the appropriate direction for each observance of a speed, if blocks run out; each number 1 can be modified to be any number greater than 1 if needed. For example in order to represent two vehicles observed at the same speed, a number 2 can be entered into a single box or two separate number 1 values can be inserted in two separate boxes. (2) The number of observations of each speed shall be summed under the Total column for each direction of travel, and the individual totals are cumulatively summed from lowest to highest speed for each direction of travel under the Cum Total column. If the electronic version of the form is used and the data is input properly, the 85 th percentile vehicle is automatically computed for each direction and shown at the bottom of the form. That vehicle s corresponding speed category represents the 85 th percentile speed. This speed is computed by interpolating two Cum Total column values (higher and lower than the 85 th percentile vehicle) using the average of the speed ranges under the Speed column. The 10 mph pace in each direction is also calculated automatically in the electronic version of the form and it displays at the bottom an OK message if a single 10 mph pace was identified. If more than one speed ranges have the highest 10 mph pace, then the highest range of speed is shown in the corresponding field and a warning message is displayed at the bottom of the spreadsheet. Vehicle Spot Speed Study 12-2

3 (3) Data can be collected as a function of vehicle classification either the Total or for both directions. This is accomplished by utilizing the classification partitions at the top of the spreadsheet to help classify data collected. Vehicles of a particular class type should be entered within the column bounds and the designated class should be noted. To help automatically summarize the data, the spreadsheet can be copied as many times as there are vehicle classes, and summaries can be computed for one given vehicle class by deleting the data for all other vehicle classes collected. This process is repeated for all vehicle classes. The engineer performing the calculations should sign the study and enter the date the calculations were completed in the space provided. (4) The number of observations or sample size can be determined based on the maximum and minimum speeds of an initial test sample size. Then this value can be used in Exhibit 9-1 of the ITE Manual of Transportation Engineering Studies, 2 nd Edition to compute the minimum sample size for a given confidence interval and permitted error. (5) Option 2 allows for the collection of speed data without the separation of speeds by direction. To do this, the observer enters a tally mark for each observance of a speed on the right side of the study form only. The number of observations of each speed is entered under the Total column for Both Directions. The individual totals are then cumulatively summed under the Cum Total column. Calculations are then made for the 85 th percentile speed and 10 mph pace and entered under Both Directions in the Speed Data Summary section. The electronic version of Form No can also be used for this option. An example of this option is shown in Figure (6) Options 3 and 4 are available by collecting speed data as a function of vehicle classification either by direction or for both directions similar to the first two options. This is accomplished by utilizing one-letter classification codes rather than the tally mark. An example of this option is shown in Figure Classification codes that may be used include the following: C = passenger car B = buses T = truck (six or more tires, single unit) M = multi-unit (semi and vehicle with trailers) (7) Any classification codes used should be noted on the study form by the observer. The one-letter code for each vehicle is inserted in a data block in the row for the appropriate speed. This allows summarization of speed data for each class of vehicle, providing that a statistically adequate number of vehicles from each class are sampled. Vehicle Spot Speed Study 12-3

4 (8) The remainder of the study form computes the 85 th percentile speed and 10 mph pace using the standard procedures identified in the Chapter 5 of the ITE Manual of Transportation Engineering Studies, 2 nd Edition. (9) The user is referred to the Manual on Speed Zoning for Highways, Roads, and Streets in Florida (FDOT Manual Number ) for further details regarding speed data collection and analysis. This manual is available from the State Traffic Engineering and Operations Office, 605 Suwannee Street, M.S. 90, Tallahassee, Florida , Phone (850) FORMS ACCESS (1) A reproducible copy of the Vehicle Spot Speed Study (Form No ) can be downloaded from the MUTS website. This form is also available from the Department's Forms and Procedures Office. Vehicle Spot Speed Study 12-4

5 Figure Advisory Speed Study Form No (Option 1) West Vehicle Spot Speed Study 12-5

6 Figure Advisory Speed Study Form No (Option 2) Vehicle Spot Speed Study 12-6

7 Figure Advisory Speed Study Form No (Option 3) Vehicle Spot Speed Study 12-7