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| [[Image:GEH Variation.png|thumb|right|A comparison of the allowable variance under the GEH formula for GEH=5 with a variance of 5 percent]]
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| The '''GEH Statistic''' is a [[formula]] used in [[Traffic engineering (transportation)|traffic engineering]], [[traffic forecasting]], and [[traffic modelling]] to compare two sets of [[traffic volume]]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 test]], is not a true [[statistics|statistical test]]. Rather, it is an [[empirical|empirical formula]] that has proven useful for a variety of traffic analysis purposes.
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| :The formula for the "GEH Statistic" is:
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| :<math>GEH=\sqrt{\frac{2(M-C)^2}{M+C}}</math>
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| :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)
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| Using the GEH Statistic avoids some pitfalls that occur when using simple [[percentage]]s to compare two sets of volumes. This is because the traffic volumes in real-world [[transportation system]]s 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]]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 [[transportation forecasting|forecasting model]]s is recognised in the UK [[Highways Agency]]'s [[Design Manual for Roads and Bridges]] (DMRB), Volume 12, Section 2, the [http://www.wisdot.info/microsimulation/index.php?title=Main_Page Wisconsin microsimulation modeling guidelines], and other references.
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| 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).
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| ==Applications==
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| The GEH formula is useful in situations such as the following:{{Citation needed|date=January 2011}}
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| * 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).
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| * 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.
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| * Comparing the traffic volumes obtained from a travel demand forecasting model (for the "base year" scenario) with the real-world traffic volumes.
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| == See also ==
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| *[[Microsimulation]]
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| *[[Traffic counter]]
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| *[[Traffic flow]]
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| *[[Traffic engineering (transportation)]]
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| *[[Transportation planning]]
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| *[[Trip generation]]
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| {{Unreferenced|date=January 2011}}
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| ==External links==
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| *[http://www.archive2.official-documents.co.uk/document/deps/ha/dmrb/index.htm UK Highways Agency's Design Manual for Roads & Bridges (DMRB)]
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| * [http://www.wisdot.info/microsimulation/index.php?title=Main_Page Wisconsin Microsimulation Modeling Guidelines]
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| [[Category:Transport engineering]]
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| [[Category:Transportation planning]]
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