A2.9 - Travel times and speed
- A2.1 - Traffic data
- A2.2 Traffic composition
- A2.3 - Separating the project into its component sections
- A2.4 - Dividing the year into time periods
- A2.5 - Vehicle occupancy and travel purpose
- A2.6 - Traffic volumes
- A2.7 - Traffic growth rates
- A2.8 - Future traffic volumes
- A2.9 - Travel times and speed
- A2.10 - References
A2.9 - Travel times and speed
Travel times and speed
Travel time and/or speeds shall be measured where required. Suitable methods for measuring average travel times or speed depending on circumstances include:
- floating car survey
- number plate survey
- spot measurement of speed.
The floating car and number plates survey methods measure the average travel time over a length of road.
The floating car survey methods is relatively cheap and convenient method but will not readily differentiate the average travel times of light and heavy traffic. It is only suitable for higher traffic volumes in excess or 500 vehicles/hour/lane.
The number plate method is a larger undertaking but potentially more accurate and has ability to give data on the average travel times of individual or categories of vehicle. Several software packages are available for analysing number plate survey data as are electronic field-book programmes for facilitating the data input.
The average travel time over a section of road may not provide sufficient information for calculating vehicle operating costs if one or more speed change cycles occur within the section. Speed change cycles should be separately identified in urban areas where speeds reduce to below 20km/h and for rural areas where vehicles slow down for example to negotiate a sharp bend or at an intersection.
In such cases, spot measurement of speed will be required at a sufficient number of other locations to establish the average cruise speed for the road section and at the points of minimum speed. If vehicles stop at any point on the road section, then the average length of stopped time will also be required for the operating cost calculations. An alternative to spot measurements of speed will be to arrange number plate survey points such that they do not contain speed change cycles within their length.
When averaging the results of speed spot measurements, the space mean speed should be calculated using the following formula:
where
vi = spot speed measurement
n = total number of spot speed measurements
