GEH
Encyclopedia
Formula
In mathematics, a formula is an entity constructed using the symbols and formation rules of a given logical language....
used in traffic engineering
Traffic engineering
Traffic engineering can mean:* traffic engineering , a branch of civil engineering* teletraffic engineering, a field of statistical techniques used in telecommunications...
, traffic forecasting, and traffic modelling to compare two sets of traffic volume
Traffic volume
In telecommunication networks, traffic volume is a measure of the total work done by a resource or facility, normally over 24 hours, and is measured in units of erlang-hours...
s. The GEH formula gets its name from Geoffrey E. Havers, who invented it in the 1970s while working as a transport planner in London, England. Although its mathematical form is similar to a chi-squared
Chi-squared
In statistics, the term chi-squared has different uses:*chi-squared distribution, a continuous probability distribution;*chi-squared statistic, a statistic used in some statistical tests;...
test, is not a true statistical test
Statistics
Statistics is the study of the collection, organization, analysis, and interpretation of data. It deals with all aspects of this, including the planning of data collection in terms of the design of surveys and experiments....
. Rather, it is an empirical formula
Empirical
The word empirical denotes information gained by means of observation or experimentation. Empirical data are data produced by an experiment or observation....
that has proven useful for a variety of traffic analysis purposes.
- The formula for the "GEH Statistic" is:
- Where M is the hourly traffic volume from the traffic model (or new count) and C is the real-world hourly traffic count (or the old count)
Using the GEH Statistic avoids some pitfalls that occur when using simple percentage
Percentage
In mathematics, a percentage is a way of expressing a number as a fraction of 100 . It is often denoted using the percent sign, “%”, or the abbreviation “pct”. For example, 45% is equal to 45/100, or 0.45.Percentages are used to express how large/small one quantity is, relative to another quantity...
s to compare two sets of volumes. This is because the traffic volumes in real-world transportation systems vary over a wide range. For example, the mainline of a freeway/motorway might carry 5000 vehicles per hour, while one of the on-ramps leading to the freeway might carry only 50 vehicle
Vehicle
A vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....
s per hour (in that situation it would not be possible to select a single percentage of variation that is acceptable for both volumes). The GEH statistic reduces this problem; because the GEH statistic is non-linear, a single acceptance threshold based on GEH can be used over a fairly wide range of traffic volumes. The use of GEH as an acceptance criterion for travel demand forecasting model
Transportation forecasting
Transportation forecasting is the process of estimating the number of vehicles or people that will use a specific transportation facility in the future. For instance, a forecast may estimate the number of vehicles on a planned road or bridge, the ridership on a railway line, the number of...
s is recognised in the UK Highways Agency
Highways Agency
The Highways Agency is an executive agency, part of the Department for Transport in England. It has responsibility for managing the core road network in England...
's Design Manual for Roads and Bridges
Design Manual for Roads and Bridges
The Design Manual for Roads and Bridges is a series of 15 volumes that provide official standards, advice notes and other documents relating to the design, assessment and operation of trunk roads, including motorways in the United Kingdom, and, with some amendments, the Republic of Ireland .They...
(DMRB), Volume 12, Section 2, the Wisconsin microsimulation modeling guidelines, and other references.
For traffic modelling work in the "baseline" scenario, a GEH of less than 5.0 is considered a good match between the modelled and observed hourly volumes (flows of longer or shorter durations should be converted to hourly equivalents to use these thresholds). According to DMRB, 85% of the volumes in a traffic model should have a GEH less than 5.0. GEHs in the range of 5.0 to 10.0 may warrant investigation. If the GEH is greater than 10.0, there is a high probability that there is a problem with either the travel demand model or the data (this could be something as simple as a data entry error, or as complicated as a serious model calibration problem).
Applications
The GEH formula is useful in situations such as the following:- Comparing a set of traffic volumes from manual traffic counts with a set of volumes done at the same locations using automation (e.g. a pneumatic tube traffic counter is used to check the total entering volumes at an intersection to affirm the work done by technicians doing a manual count of the turn volumes).
- Comparing the traffic volumes obtained from this year's traffic counts with a group of counts done at the same locations in a previous year.
- Comparing the traffic volumes obtained from a travel demand forecasting model (for the "base year" scenario) with the real-world traffic volumes.
See also
- MicrosimulationMicrosimulation-Introduction:Microsimulation is a category of computerized analytical tools that perform highly detailed analysis of activities such as highway traffic flowing through an intersection, financial transactions, or pathogens spreading disease through a population...
- Traffic counter
- Traffic flowTraffic flowTraffic flow, in mathematics and civil engineering, is the study of interactions between vehicles, drivers, and infrastructure , with the aim of understanding and developing an optimal road network with efficient movement of traffic and minimal traffic congestion problems.-History:Attempts to...
- Traffic engineeringTraffic engineeringTraffic engineering can mean:* traffic engineering , a branch of civil engineering* teletraffic engineering, a field of statistical techniques used in telecommunications...
- Transportation planningTransportation planningTransportation planning is a field involved with the evaluation, assessment, design and siting of transportation facilities .-Models and Sustainability :...
- Trip generationTrip generationTrip generation is the first step in the conventional four-step transportation forecasting process , widely used for forecasting travel demands...