HCM2010MetricAnalysisGuidelines
Prepared by the User Liaison subcommittee (2/20/2012) Purpose
The HCM 2010 is the fifth major edition of the Highway Capacity Manual. Unlike the 2000 HCM produced in both US Customary and metric units, the latest manual is written solely in US Customary units. This includes input parameters, coefficients in formulas, and computational engines (where no metric versions have been developed). Therefore, some general guidelines for performing a metric analysis using the HCM 2010 procedures are appropriate. SoftConversionversusHardConversion
Because of the methodology constraints resulting from selection of US Customary units, users with metric designs must convert input data into US Customary units prior to analysis. This should be done primarily through soft conversion. Soft conversion means the metric input values are converted into US Customary units without rounding, using standard conversion multipliers, which are then used in the analysis. Output measures will be in US Customary units, which will then require conversion back into metric units for reporting results. While conversion from metric to US Customary units is straightforward, there will be instances where some rounding is needed. One place where this could occur is with lane widths. Metric lane widths, when converted to US Customary widths, may require the analyst to input a whole unit for the analysis. This is referred to as a hard conversion. For example: 3.6 m  3.281 ft/m = 11.81 ft  round up to 12 ft The user would then input 12 feet as the lane width into the analysis. This is saying the 3.6 m lane width is operationally equivalent to a 12‐ft lane width. The appropriate conversions are shown in Table 1 for both lane width and shoulder width/lateral clearance, based off of the standard US Customary value. Table 1. (a) Lane Width Conversions, (b) Shoulder Width/Lateral Clearance Conversions (a) (b) Lane Width Shoulder Width/Lateral Clearance US Customary (ft) Metric (m) US Customary (ft) Metric (m) 12 3.6 10 3.0 11 3.3 8 2.4 10 3.0 6 1.8 9 2.7 4 1.2 2 0.6 Note that hard conversion inputs, as shown in Table 1, should only be used when the input parameter is expected to be a whole number in the methodology. The user should refer to the HCM 2010 for guidance on the expected input values and determine for themselves whether hard or soft conversion is most appropriate1. Another parameter where hard conversion may be appropriate is free flow speed (FFS), or speed limit. Where an analysis requires the input of a whole value of speed, the metric value must be equated to a US Customary input value. Table 2 provides the appropriate conversion. Table 2. Speed Conversion from US Customary to Metric (1 mi/h = 1.609 km/h) US Customary (mi/h) Soft Conversion (km/h) Hard Conversion (km/h) 25 40.2 40 30 48.3 50 35 56.3 55 40 64.4 65 45 72.4 70 50 80.5 80 55 88.5 90 60 96.5 95 65 104.6 105 70 112.6 110 75 120.7 120 ComparingResultstothe2000HCM
Users are cautioned concerning direct comparison with results obtained from the HCM 2000. The metric version of that manual was a complete hard conversion edition. Methodologies and graphs were converted and created in metric units. With the 2010 HCM, conversion of metric inputs into US Customary units and then running the analysis will produce slightly different results when compared to the metric HCM 2000. Further, many of the methodologies in the 2010 HCM are completely different than the previous version (e.g., freeway weaving, signalized intersections, urban streets, and roundabouts). ServiceMeasures
Users in the metric community are encouraged to use soft conversion to describe the threshold values for a specific service measure of effectiveness. This will facilitate communication with other users and decision makers on a common basis. Soft conversion also enables another person replicating the calculations using soft conversion to obtain the same result. Users are also cautioned when results show values near the boundary between levels of service for a specific service measure. Because some input parameters may have been converted using a hard conversion (like lane width or speed), the results may show one level of service when the adjacent level of service is more accurate. Results within the middle of the service measure range are usually not affected by hard conversion versus soft conversion. 1
Note that p. 1‐6 of the HCM 2010 states “…a soft conversion is favored from U.S. customary units to metric units, so that computational engines produce the same result regardless of the measurement system used.” CommonConversions
Throughout the manual, various input or output parameters will need conversion from one system to the other. The following table shows the appropriate unit designation for many of these parameters along with the conversion factor. Table 3. Recommended Unit Conversions Quantity Length Time Traffic Lane Person or Vehicle Mass Weight Power Engine Size Flow Rate Saturation Flow Rate Total Travel Delay Density Speed Load Factor Space Headway Precipitation Rate US Customary Unit inch foot mile day hour minute second lane person or pedestrian vehicle passenger car bus slug pound horsepower cubic inch vehicles per hour pedestrians per hour persons per vehicle buses per hour passenger cars per hour passenger cars per hour per lane vehicle‐miles seconds per vehicle passenger cars per mile per lane miles per hour persons per seat square feet per pedestrian seconds per vehicle inches per hour U.S. Customary Symbol in ft mi day h min s ln p veh pc bus slug lb hp in3 veh/h p/h p/veh buses/h pc/h pc/h/ln veh‐mi s/veh pc/mi/ln mi/h p/seat ft2/p s/veh in/h Metric Unit millimeter meter kilometer day hour minute second lane person or pedestrian vehicle passenger car bus kilogram newton kilowatt cubic centimeter vehicles per hour pedestrians per hour persons per vehicle buses per hour passenger cars per hour passenger cars per hour per lane vehicle‐kilometers seconds per vehicle passenger cars per kilometer per lane kilometer per hour persons per seat square meter per pedestrian seconds per vehicle millimeters per hour Metric Symbol mm m km day h min s ln p veh pc bus kg N kW cm3 veh/h p/h p/veh bus/h pc/h pc/h/ln Conversion Factor from U.S. to Metric 25.4 mm/in 0.3048 m/ft 1.609 km/mi na na na na na na na na na 14.59 kg/slug 4.448 N/lb 0.746 kW/hp 16.387 cm3/in3 na na na na na na veh‐km s/veh pc/km/ln 1.609 km/mi na 0.621 mi/km km/h p/seat m2/p 1.609 km/h na 0.0929 m2/ft2 s/veh mm/h na 25.4 mm/in Disclaimer
These guidelines are not an official supplement to the 2010 HCM, nor endorsed by the Transportation Research Board. These guidelines were independently developed within the User Liaison Subcommittee of the TRB Highway Capacity and Quality of Service Committee as guidance to the metric user community in using the 2010 HCM. Users are solely responsible for interpreting and converting inputs and outputs in metric units and in reporting results.