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Incorporating Travel Time Reliability into the Highway Capacity Manual (2014)

Chapter: Appendix G - Freeway Free-Flow Speed Adjustments for Weather, Incidents, and Work Zones

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Suggested Citation:"Appendix G - Freeway Free-Flow Speed Adjustments for Weather, Incidents, and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Travel Time Reliability into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22487.
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Suggested Citation:"Appendix G - Freeway Free-Flow Speed Adjustments for Weather, Incidents, and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Travel Time Reliability into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22487.
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Suggested Citation:"Appendix G - Freeway Free-Flow Speed Adjustments for Weather, Incidents, and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Travel Time Reliability into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22487.
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Suggested Citation:"Appendix G - Freeway Free-Flow Speed Adjustments for Weather, Incidents, and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Travel Time Reliability into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22487.
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Suggested Citation:"Appendix G - Freeway Free-Flow Speed Adjustments for Weather, Incidents, and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Travel Time Reliability into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22487.
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Suggested Citation:"Appendix G - Freeway Free-Flow Speed Adjustments for Weather, Incidents, and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Travel Time Reliability into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22487.
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Suggested Citation:"Appendix G - Freeway Free-Flow Speed Adjustments for Weather, Incidents, and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Travel Time Reliability into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22487.
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Suggested Citation:"Appendix G - Freeway Free-Flow Speed Adjustments for Weather, Incidents, and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Travel Time Reliability into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22487.
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Suggested Citation:"Appendix G - Freeway Free-Flow Speed Adjustments for Weather, Incidents, and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Travel Time Reliability into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22487.
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216 This appendix presents recommended free-flow speed adjust- ment factors (SAFs) for weather. The recommendations are based on a review of the literature and extraction of relevant data found in the literature. HCM Definitions This section presents the 2010 Highway Capacity Manual (HCM2010) (Transportation Research Board of the National Academies 2010) definitions and values for freeway free-flow speed and capacity. Free-Flow Speed Chapter 10 of the HCM2010 defines free-flow speeds on free- ways as “[t]he theoretical speed when the density and flow rate on the study segment are both zero. Chapter 11, Basic Freeway Segments, presents speed–flow curves that indicate that the free-flow speed on freeways is expected to prevail at flow rates between 0 and 1,000 passenger cars per hour per lane (pc/h/ln). In this broad range of flows, speed is insensitive to flow rates.” The free-flow speeds for dry pavement, fair weather, non- incident conditions define the base capacity for the freeway according to Exhibit 10-5 of the HCM2010. The relationship between free-flow speed and freeway base capacity is given in Table G.1. The equivalent equation is given by Equation G.1: Base Capacity pc h ln 2,400 pc h ln 10 70 min 70,FFS (G.1) ( ) ( )( ) = − × − where FFS = free-flow speed under dry pavement, fair weather, nonincident conditions (mph). Capacity and Speed at Capacity Exhibit 11-2 in the HCM2010 defines capacity when traffic is at a density of 45 passenger cars per mile per lane for basic freeway segments under clear weather, dry pavement, non- incident conditions. The speed at capacity can then be derived from this information by using the basic speed–flow–density relationship. The speeds at capacity for different free-flow speeds are given in Table G.2. The equivalent equation for the entries in this table is given by Equation G.2: Speed at Capacity mph 2,400 pc h ln 10 70 min 70,FFS 45 (G.2)[ ] ( ) ( )( )= − × − HCM Freeway Speed–Flow Curves The clear weather, dry pavement speed–flow curves for basic freeway segments shown in Exhibit 11-2 of the HCM2010 can be approximated using the equations given in Exhibit 11-3 and shown here in Equation G.3: FFS if ; otherwise, FFS (G.3) 2 S v BP S A v BP p p( ) = < = − × − where S = speed at passenger car equivalent volume vp (mph); A = calibration parameter (see Table G.3); B = breakpoint passenger car equivalent volume (pc/h/ln); P = 1,000 + 200 × (75 - FFS) / 5; and V = passenger car equivalent volume (pc/h/ln). Equation G.3, however, does not provide for adjustments to the dry weather, nonincident capacity that can occur with bad weather or incidents. Equation 25-1 from Chapter 25 of the HCM2010 (shown here as Equation G.4) applies: FFS 1 (G.4) ln FFS 1 CAF 45 CAFS e C v C p = + −     ( )+ − × ×  A p p e n D i x G Freeway Free-Flow Speed Adjustments for Weather, Incidents, and Work Zones

217 where S = segment speed (mph); FFS = segment free-flow speed (mph); C = original segment capacity (pc/h/ln); CAF = capacity adjustment factor (unitless), subject to CAF > 0 and CAF < 45 × (FFS + 1)/C; and vp = segment flow rate (pc/h/ln). Although HCM2010 Equation 25-1 is not precisely flat for passenger car equivalent volumes under 1,000 passenger cars per hour per lane (pcphpl), it is close enough for the purposes of speed and travel time prediction, and it has the advantage of being sensitive to capacity adjustments for weather, inci- dents, and work zones. With a slight modification (the addition of a free-flow SAF to account for weather effects), HCM2010 Equation 25-1 can be used to predict speeds for weather, as well as incidents and work zones. This modification is shown in Equation G.5: FFS FAF 1 (G.5) ln FFS FAF 1 CAF 45 CAFS e C v C p = × + −     ( )× + − × × × where all variables are the same as in Equation G.4, with the addition of FAF, the free-flow speed adjustment factor (unit- less), which is subject to CAF > 0 and CAF < 45 × (FFS × FAF + 1) / C, and FAF > (C × CAF / 45 - 1) / FFS. HCM Capacity Adjustments The HCM capacity adjustments for weather, incidents, and work zones must be examined to ensure that the recom- mended free-flow SAFs do not fall below the limits set by Equation G.5. Weather Capacity Adjustments Exhibit 10-15 of the HCM2010 provides ranges and average capacity adjustments by weather type, based on research on Iowa freeways. This exhibit is shown as Table G.4. The impli- cations for the minimum allowable free-flow SAF are shown in the right-hand columns of this table for freeways with dry weather free-flow speeds between 55 and 75 mph. Two extrap- olations of the original HCM exhibit have been included here for weather conditions not explicitly covered in the origi- nal exhibit. Capacity adjustment factors (CAFs) for wet pave- ment, clear weather conditions have been set equivalent to light rain conditions. The capacity for light wind (<10 mph) conditions has been set equal to that for clear, dry pavement conditions (CAF = 1.00). CAFs are applied to the base capacity as shown in Equation G.6: Base Capacity Weather Base Capacity Clear, Dry CAF (G.6) ( )( ) = × where Base Capacity (Weather) = Base capacity for inclement weather (pc/h/ln); Table G.1. Relationship Between Free-Flow Speed and Freeway Base Capacity Free-Flow Speed (mph)a Base Capacity (pc/h/ln) 75 2,400 70 2,400 65 2,350 60 2,300 55 2,250 Source: HCM2010, Exhibit 10-5. a Dry pavement, fair weather, nonincident. Table G.2. Dry Weather Speed at Capacity for Different Free-Flow Speeds Free-Flow Speed (mph) Capacity (pc/h/lane)a Density at Capacity (pc/mi/ln)b Speed at Capacity (mph) 75 2,400 45 53.3 70 2,400 45 53.3 65 2,350 45 52.2 60 2,300 45 51.1 55 2,250 45 50.0 Source: Computed from Exhibit 10-5, 2010 HCM. a pc/h/lane = passenger cars per hour per lane. b pc/mi/lane = passenger cars per mile per lane. Table G.3. HCM2010 Values for “A” Parameter in Freeway Free-Flow Speed Equations FFS A 75 mph 1.107 × 10-5 70 mph 1.160 × 10-5 65 mph 1.418 × 10-5 60 mph 1.816 × 10-5 55 mph 2.469 × 10-5 Source: Exhibit 11-3, HCM2010.

218 Base Capacity (Clear, Dry) = Base capacity for dry pave- ment, fair weather, non- incident conditions (pc/h/ ln); and CAF = capacity adjustment factor (unitless) (see Table G.5). Incident Capacity Adjustments Exhibit 10-17 of the HCM2010 provides recommended CAFs for incidents (see Table G.5). The HCM CAFs are for the entire facility for differing numbers of lanes before and during the incident. These fac- tors need to be translated into capacity per lane values for the lanes remaining open during the incident in order to be able to determine the appropriate minimum values for the free- flow SAFs. Table G.6 shows the CAFs in a capacity per open lane format after the conversion. Table G.7 shows the minimum allowable free-flow SAFs for incidents on a freeway with a 55-mph free-flow speed and a base capacity of 2,250 pcphpl. Work Zone Capacity Adjustments Work zones include short-term work zone lane closures due to maintenance and long-term lane closures due to construc- tion. According to the Manual on Uniform Traffic Control Devices (Federal Highway Administration 2009), construction duration for long-term work zones is more than 3 days and could last several weeks, months, or even years, depending on Weather Type Capacity Adjustment Factors Minimum Allowable Free-Flow Speed Adjustment Factors(According to Freeway Free-Flow Speed) Low High Ave 55 mph 60 mph 65 mph 70 mph 75 mph Clear Dry Pavement 1.00 1.00 1.00 0.89 0.84 0.79 0.75 0.70 Wet Pavement* 0.96 0.99 0.98 0.88 0.83 0.78 0.74 0.69 Rain 0.96 0.99 0.98 0.88 0.83 0.78 0.74 0.69 0.90 0.94 0.93 0.84 0.78 0.74 0.70 0.66 > 0.25 in/h 0.82 0.89 0.86 0.79 0.74 0.70 0.66 0.62 Snow 0.94 0.96 0.96 0.85 0.80 0.76 0.72 0.67 0.88 0.94 0.91 0.84 0.78 0.74 0.70 0.66 0.87 0.92 0.89 0.82 0.77 0.72 0.69 0.64 > 0.50 in/h 0.72 0.79 0.78 0.70 0.66 0.62 0.59 0.55 Temp < 50 deg F 0.99 0.99 0.99 0.88 0.83 0.78 0.74 0.69 < 34 deg F 0.98 0.98 0.98 0.87 0.82 0.77 0.73 0.68 < –4 deg F 0.90 0.93 0.91 0.83 0.78 0.73 0.69 0.65 Wind < 10 mph* 1.00 1.00 1.00 0.89 0.84 0.79 0.75 0.70 0.99 0.99 0.99 0.88 0.83 0.78 0.74 0.69 > 20 mph 0.98 0.99 0.98 0.88 0.83 0.78 0.74 0.69 Visibility < 1 mi N/A N/A 0.93 0.83 0.78 0.73 0.69 0.65 N/A N/A 0.88 0.78 0.73 0.69 0.66 0.61 N/A N/A 0.89 0.79 0.74 0.70 0.66 0.62 Source: Exhibit 10-15, 2010 HCM (TRB 2010). * Weather categories extrapolated as explained in text. N/A = not applicable, data not available. 0.10 in/h≤ 0.25 in/h≤ 0.05 in/h≤ 0.10 in/h≤ 0.50 in/h≤ 20 mph≤ 0.50 mi≤ 0.25 mi≤ Table G.4. Weather Adjustments to Freeway Base Capacity

219 the nature of works. Short-term work zone duration is more than an hour and within a single daylight period (Federal Highway Administration 2009). Long-term construction zones generally use portable concrete barriers, while short-term work zones use standard channelizing devices. Chapter 10 of the HCM2010 summarizes the lane closures and ranges of capacity during construction. Exhibit 10-14 of the HCM2010 provides work zone capacities in terms of vehicles per hour per lane according to the original number of lanes (before the work zone) and the number of lanes open when the work zone is in place. In Table G.8, the passenger car per hour per lane equivalent is computed assuming level terrain, 5% heavy vehicles, and a 0.90 peak hour factor. The vehicle per hour per lane capacities in Exhibit 10-14 of HCM2010 were converted to passenger car equivalents for the purpose of computing CAFs for work zones. CAFs for a free- way with a 65-mph free-flow speed were computed assuming that the values in Figure 10-14 of the HCM2010 apply to a freeway with a 65-mph free-flow speed and a base condition of dry weather and nonwork zone capacity of 2,300 pcphpl. The same CAFs computed for a freeway with a 65-mph Number of Lanes ) Shoulder Disablement Shoulder Accident One Lane Blocked Two Lanes Blocked Three Lanes Blocked 2 0.95 0.81 0.35 0.00 N/A 3 0.99 0.83 0.49 0.17 0.00 4 0.99 0.85 0.58 0.25 0.13 5 0.99 0.87 0.65 0.40 0.20 6 0.99 0.89 0.71 0.50 0.26 7 0.99 0.91 0.75 0.57 0.36 8 0.99 0.93 0.78 0.63 0.41 Source: Exhibit 10-17, HCM2010. N/A = not applicable, scenario not feasible. Table G.5. Capacity Adjustment Factors According to “Before Incident” Conditions N/A = not applicable, data not available. Table G.6. Open Lane Capacity Adjustment Factors for Incidents N/A N/A N/A N/A N/A N/A N/A N/A N/A = not applicable, data not available. Table G.7. Minimum Free-Flow Speed Adjustment Factors for 55-mph Freeways

220 free-flow speed are assumed to apply to freeways with higher and lower free-flow speeds. In other words, the effect of the work zone on capacity is assumed to be proportional to the base capacity. The resulting CAFs applicable to all freeways, regardless of free-flow speed, are shown in the second col- umn from the left in Table G.9. Exhibit 10-14 of the HCM2010 has been extra polated to freeway work zones with five moving lanes. The right-hand five columns of Table G.9 show the equivalent minimum free-flow SAFs con- sistent with the computed CAFs. Literature on Speed effects Weather Effects During adverse weather—such as rain or snow—drivers usu- ally slow down systemwide due to lower visibility and wet, icy, or slushy pavement conditions. Depending on the intensity of the rain or snow event, the speed adjustment can be little, noticeable, or significant. Researchers around the world have studied the effect of severe weather on free-flow speed. Their findings and average speed reductions calculated from the literature summary are presented in Table G.10. The low end of the reduction ranges could be applicable to light adverse weather or free-flow conditions, while the higher numbers could be applied to roadways with at-capacity volumes or under heavy rain or snow. Strong et al. (2010) also conducted a thorough literature review on the topic. Among their most relevant findings is a study done by Japanese researchers, who found a 15% speed reduction for a blizzard condition, 18.6% for frozen pave- ment, 6.5% for snow flurries and snowfall, 6% for wet pave- ment, 11.3% for melting snow, 12% to 44% for compacted snow, and 15.4% for icy conditions. Incident Effects Data and literature on the effects of incidents on free-flow speeds are relatively rare and were not encountered in the limited literature research conducted for this appendix. Work Zone Effects The effects of work zones on free-flow speeds have not been examined in the literature. However, the effectiveness of work zone speed limits at reducing free-flow speeds within the work zone has been examined for different levels and methods of posting and enforcement. Original Number of Lanes Note: N/A = not applicable, data not available. Source: Default values and ranges from Exhibit 10-14, HCM2010. Table G.8. Capacities of Freeway Work Zones Note: The minimum allowable free-flow speed adjustment factors are according to base free-flow speed and base capacity. CAF = capacity adjustment factor. Table G.9. Capacity and Minimum Free-Flow Speed Adjustment Factors for Work Zones

221 This section summarizes past research efforts performed on work zone and speed and enforcement. Richards et al. (1985) studied several work zone speed control methods. Their study results indicate that flagging and law enforcement are effective methods for controlling speeds at work zones. The flagging treatment tested reduced speed an average of 19% for all sites, and the law enforcement treatment reduced speed an average of 18%. Wasson et al. (2011) evaluated the temporal and spatial effects of work zone speed limit compliance over short 1- and 2-mi segments, as well as for the overall 12.2-mi work zone and approaching transition areas. Space mean speed was measured for approximately 11% of passing vehicles using 13 Bluetooth probe data acquisition stations. The presence of enforcement activities resulted in statistically significant reductions in the space mean speeds in the areas of enforce- ment and the adjacent highway segments. Although space mean speed was reduced by approximately 5 mph over the 12.2-mi segment during the enforcement activity, within 30 min of suspending the enforcement detail, the space mean speed increased and there was no statistically significant resid- ual impact on the space mean speed for the 12.2-mi segment. Hou et al. (2011) conducted field studies on three I-70 main- tenance short-term work zones in rural Missouri for three speed limit scenarios: no posted speed limit reduction, a 10-mph posted speed limit reduction, and a 20-mph posted speed limit reduction. The observed 85th percentile speeds were 81, 62, and 48 mph for no posted speed limit reduction, a 10-mph posted speed limit reduction, and a 20-mph posted speed limit reduction, respectively. The percentage of drivers who exceeded the posted speed limit by over 10 mph were 15.4%, 4.8%, and 0.9% with no speed limit reduction, a 10-mph posted speed limit reduction, and a 20-mph posted speed limit reduction, respectively. Researchers concluded that a reduction in posted speed limit was effective in reducing prevailing speeds in Missouri. Brewer et al. (2005) tested three devices: speed display trailer, changeable message sign with radar, and orange-border speed limit sign. They found that devices that display an approaching driver’s speed are effective at reducing speed and improving work zone speed compliance. In the absence of active work tak- ing place and when the road maintains a normal cross section, drivers generally maintain the speed they were traveling before entering the work zone, regardless of the posted work zone speed limit. Officials should post the realistic speed limit to avoid work zone speed limits that drivers ignore or widely dis- obey, and the speed limits should be confined as much as possible to the specific areas where active work is taking place. Franz and Chang (2011) evaluated the effectiveness of an automated speed enforcement system in work zones. Before versus during enforcement periods analysis showed a general reduction in speeding by aggressive motorists, while creating a more stable speeding distribution through the work zone. The comparison of during versus after enforcement periods showed that motorists may learn where enforcement is taking place and adjust their speed accordingly. Li et al. (2010) evaluated the effectiveness of a portable changeable message sign (PCMS) in reducing vehicle speeds Weather Type Researchers Location Rainfall Wet Pavement Snowfall Icy Pavement Kilpelainen and Summala (2007) Finland 6–7 km/h Koetse and Rietveld (2009) N/A N/A Up to 25% N/A Up to 25% Martin et al. (2000) Utah (Arterials) 10% 13% 13% 30% HraN/Acet al. (2006) 3% a,b a 9% b,c N/A 5% c N/A Maze et al. (2006) Minneapolis United States United States 6% N/A 13% N/A Sabir et al. (2008) the Netherlands 10–15% d N/A 7% N/A Strong et al. (2010) N/A N/A N/A 6 mphc 31 mph f N/A Rakha et al. (2008) 3–6% b,e 8–10% c,e 6–9% b,f 8–14% c,d N/A 5–16% b,e 5–16% c,e 5–19% b,e N/A Goodwin (2002) 10–25% 30–40% 10–25% 30–40% Padget et al. (2001) Iowa (Arterial) N/A N/A N/A 18–20% Average 7–11% 19–21% 9–12% 22–24% Note: This table shows results for arterials as well as freeways. a Under adverse weather and road conditions. b Free-flow. c At capacity. d Rush. e Light. f Heavy. N/A = not applicable, data not available. Table G.10. Comparison and Summary of Literature Findings on Speed Reduction due to Weather

222 in the upstream of one-lane, two-way work zones on rural highways. The evaluation was performed under three condi- tions during field experiments: PCMS switched on, PCMS switched off but still visible, and PCMS removed from the road and out of sight. The results indicated that the PCMS, whether turned on or off, was significantly more effective than the PCMS absent from the highway. Vehicle speeds were reduced by 4.7 mph and 3.3 mph when the PCMS was turned on and off, respectively. When the PCMS was absent from the road, the speed reduction was 1.9 mph. Hajbabaie et al. (2009) compared the effects of four speed management techniques: speed feedback trailer, police car, speed feedback trailer plus police car, and automated speed photo-radar enforcement. All the law enforcement methods significantly reduced the mean speed of free-flowing cars by 6.1 to 8.4 mph in the median lane and by 4.2 to 6.9 mph in the shoulder lane. In the moderately speeding site, police and speed photo-radar enforcement reduced the mean speeds similarly in both lanes; however, trailer plus police car treat- ment resulted in even larger speed reductions. Theiss et al. (2010) conducted a study on the operational effectiveness of electronic speed limit signs and flexible roll- up work zone speed limit signs. Researchers concluded from the long-term field study that motorists understood and appreciated the intent of the electronic speed limit signs. The short-term field study of both the electronic speed limit and flexible roll-up work zone speed limit signs resulted in lower mean speeds and percentage of vehicles exceeding the speed limit downstream of the reduced work zone speed limit compared with standard temporary speed limit signing. The researchers recommended the use of electronic and flexible roll-up work zone speed limit signs to better manage short- term speed limits because of the simplicity this signage offers in varying speed limits to match conditions. Recommended Free-Flow Speed Adjustments This section presents the recommended freeway free-flow speed adjustments for the effects of weather, incidents, and work zones. Weather Free-Flow Speed Adjustments Based on the preceding information, the free-flow speed reductions shown in Table G.11 are recommended for adverse weather conditions on urban and rural freeways. The weather categories in Table G.11 are adapted from Exhibit 10-15 of the HCM2010. The free-flow speed at base condition is under clear weather, dry pavement, and nonincident conditions. All Weather Type Clear Weather, Dry Pavement Free-Flow Speeds 55 mph 60 mph 65 mph 70 mph 75 mph Clear Dry Pavement 1.00 1.00 1.00 1.00 1.00 Wet Pavement 0.97 0.96 0.96 0.95 0.94 Rain Ä 0.10 in/h 0.97 0.96 0.96 0.95 0.94 Ä 0.25 in/h 0.96 0.95 0.94 0.93 0.93 > 0.25 in/h 0.94 0.93 0.93 0.92 0.91 Snow Ä 0.05 in/h 0.94 0.92 0.89 0.87 0.84 Ä 0.10 in/h 0.92 0.90 0.88 0.86 0.83 Ä 0.50 in/h 0.90 0.88 0.86 0.84 0.82 > 0.50 in/h 0.88 0.86 0.85 0.83 0.81 Temp < 50 deg F 0.99 0.99 0.99 0.98 0.98 < 34 deg F 0.99 0.98 0.98 0.98 0.97 < -4 deg F 0.95 0.95 0.94 0.93 0.92 Wind < 10 mph 1.00 1.00 1.00 1.00 1.00 Ä 20 mph 0.99 0.98 0.98 0.97 0.96 > 20 mph 0.98 0.98 0.97 0.97 0.96 Visibility < 1 mi 0.96 0.95 0.94 0.94 0.93 Ä 0.50 mi 0.95 0.94 0.93 0.92 0.91 Ä 0.25 mi 0.95 0.94 0.93 0.92 0.91 Table G.11. Recommended Freeway Free-Flow Speed Adjustment Factors for Weather

223 the recommended free-flow SAFs equal or exceed the mini- mum values given in Exhibit 10-15 of the HCM2010. Rakha et al. (2008), who produced one of the few papers to isolate free-flow speed effects from capacity speed effects, were the primary source for the free-flow speed adjustments in Table G.11. The higher end of the range of percentage adjust- ments was assumed to apply to the freeways with the highest free-flow speeds under clear weather, dry pavement conditions. Their heavy rain and heavy snowfall adjustments were assumed to apply to the highest levels of rainfall and snowfall in the Iowa study cited in Exhibit 10-15 of the HCM2010. Their light rain and light snow adjustments were assumed to apply to the lowest rainfall and snowfall categories in Exhibit 10-15. Speed adjustment values for intermediate rainfall and snowfall rates were interpolated between their high and low values. The free-flow speed for any weather categories can be derived by multiplying the clear weather, dry pavement free-flow speed for the facility by the free-flow SAF for the appropriate weather event in Table G.11. Incident Free-Flow Speed Adjustments Due to the lack of data on free-flow speeds in incident zones, it is recommended that a nominal free-flow SAF of 1.00 be used. It may be reduced at the discretion of the analyst to reflect the possible effects of rubbernecking. Work Zone Free-Flow Speed Adjustments The effects on free-flow speeds of narrower lanes and reduced right-side lateral clearances within the work zone (see Chap- ter 11 of the HCM2010) are presumed to be accounted for in the selected reduced posted speed limit for the work zone. The work zone free-flow speed is then the before work free- flow speed adjusted for changes in the posted speed limit through the work zone. The effectiveness of the work zone speed limit at reducing free-flow speed is discounted accord- ing to the degree of visibility of the speed limits and the degree of enforcement within the work zone. The computation is shown by Equation G.7: FFS FFS PSL PSL F (G.7)WZ HCM WZ NWZ ENF[ ]= + − × where FFSWZ = Free-flow speed within the work zone; FFSHCM = Geometrically determined free-flow speed com- puted or field-measured per HCM; PSLWZ = Posted speed limit within the work zone; PSLNWZ = Posted speed limit without the work zone; and FENF = Enforcement adjustment factor to account for the effects of different levels of signing and enforce- ment of the work zone speed limit. Based on the literature, the enforcement adjustment factors shown in Table G.12 are recommended. The free-flow SAF for the work zone is then the estimated work zone free-flow speed divided by the before work zone free-flow speed. If there is no change in the posted speed limit for the facil- ity within the work zone, then there is no change in the free- flow speed within the work zone. The free-flow SAF in this case is 1.00. References Brewer, M. A., G. Pesti, and W. Schneider. Identification and Testing of Measures to Improve Work Zone Speed Limit Compliance. Report FHWA/TX-60/0-4707-1. Federal Highway Administration, Washington, D.C., 2005. Federal Highway Administration. Manual on Uniform Traffic Control Devices. Washington, D.C., 2009. Franz, M. L., and G. Chang. Effects of Automated Speed Enforcement in Maryland Work Zones. Presented at 90th Annual Meeting of the Transportation Research Board, Washington, D.C., 2011. Goodwin, L. C. Weather Impacts on Arterial Traffic Flow. Mitretek Systems, Inc., Falls Church, Va., 2002. Hajbabaie, A., R. F. Benekohal, M. Chitturi, M. Wang, and J. Medina. Comparison of Automated Speed Enforcement and Police Presence on Speeding in Work Zones. Presented at 88th Annual Meeting of the Transportation Research Board, Washington, D.C., 2009. Hou, Y., P. Edara, and C. Sun. Speed Limit Effectiveness in Short-Term Rural Interstate Work Zones. Presented at 90th Annual Meeting of the Transportation Research Board, Washington, D.C., 2011. Hranac, R., E. Sterzin, D. Krechmer, H. Rakha, and M. Farzaneh. Empir­ ical Studies on Traffic Flow in Inclement Weather. FHWA-HOP-07-073. Federal Highway Administration, Washington, D.C., 2006. Kilpelainen, M., and H. Summala. Effects of Weather and Weather Fore- casts on Driver Behaviour. Transportation Research Part F, Vol. 10, No. 4, 2007, pp. 288–299. Enforcement Measure Enforcement Adjustment Factor (%) Static signs 50 Flagmen 70 Dynamic feedback signs 80 Visibly present enforcement personnel 90 Feedback signs plus visibly present enforcement personnel 100 Table G.12. Enforcement Adjustment Factors for Work Zone Free-Flow Speeds

224 Koetse, M. J., and P. Rietveld. The Impact of Climate Change and Weather on Transport: An Overview of Empirical Findings. Trans­ portation Research Part D, Vol. 14, No. 3, 2009, pp. 201–205. Li, Y., Y. Bai, and U. Firman. Determining the Effectiveness of PCMS on Reducing Vehicle Speed in Rural Highway Work Zones. Presented at 89th Annual Meeting of the Transportation Research Board, Washington, D.C., 2010. Martin, P., J. Perrin, B. Hansen, and I. Quintana. Inclement Weather Sig­ nal Timings. UTL Research Report MPC01-120. Utah Traffic Lab, University of Utah, Salt Lake City, 2000. Maze, T. H., M. Agarwal, and G. Burchett. Whether Weather Matters to Traffic Demand, Traffic Safety, and Traffic Operations and Flow. In Transportation Research Record: Journal of the Transportation Research Board, No. 1948, Transportation Research Board of the National Academies, Washington, D.C., 2006, pp. 170–176. Padget, E. D., K. K. Knapp, and G. B. Thomas. Investigation of Winter- Weather Speed Variability in Sport Utility Vehicles, Pickup Trucks, and Passenger Cars. In Transportation Research Record: Journal of the Transportation Research Board, No. 1779, TRB, National Research Council, Washington, D.C., 2001, pp. 116–124. Rakha, H., M. Farzaneh, M. Arafeh, and E. Sterzin. Inclement Weather Impacts on Freeway Traffic Stream Behavior. In Transportation Research Record: Journal of the Transportation Research Board, No. 2071, Transportation Research Board of the National Acade- mies, Washington, D.C., 2008, pp. 8–18. Richards, S. H., R. C. Wulderlich, and C. Dudek. Field Evaluation of Work Zone Speed Control Techniques. In Transportation Research Record 1035, TRB, National Research Council, Washington, D.C., 1985, pp. 66–78. Sabir, M., J. Van Ommeren, M. J. Koetse, and P. Rietveld. Welfare Effects of Adverse Weather Through Speed Changes in Car Commuting Trips. Tinbergen Institute Discussion Paper 2008-087/3. VU Uni- versity, Amsterdam, Netherlands, 2008. Strong, C. K., Z. Ye, and X. Shi. Safety Effects of Winter Weather: The State of Knowledge and Remaining Challenges. Transport Reviews, Vol. 30, No. 6, 2010, pp. 677–699. Theiss, L., M. D. Finley, and N. D. Trout. Devices to Implement Short- Term Speed Limits in Texas Work Zones. In Transportation Research Record: Journal of the Transportation Research Board, No. 2169, Transportation Research Board of the National Academies, Wash- ington, D.C., 2010, pp. 54–61. Transportation Research Board. Highway Capacity Manual 2010. TRB of the National Academies, Washington, D.C., 2010. Wasson, J. S., G. W. Boruff, A. M. Hainen, S. M. Remias, E. A. Hulme, G. D. Farnsworth, and D. M. Bullock. Evaluation of Spatial and Tem- poral Speed Limit Compliance in Highway Work Zones. In Trans­ portation Research Record: Journal of the Transportation Research Board, No. 2258, Transportation Research Board of the National Academies, Washington, D.C., 2011, pp. 1–15.

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TRB’s second Strategic Highway Research Program (SHRP 2) Report S2-L08-RW-1: Incorporation of Travel Time Reliability into the Highway Capacity Manual presents a summary of the work conducted during the development of two proposed new chapters for the Highway Capacity Manual 2010 (HCM2010). These chapters demonstrated how to apply travel time reliability methods to the analysis of freeways and urban streets.

The two proposed HCM chapters, numbers 36 and 37, introduce the concept of travel time reliability and offer new analytic methods. The prospective Chapter 36 for HCM2010 concerns freeway facilities and urban streets, and the prospective supplemental Chapter 37 elaborates on the methodologies and provides an example calculation. The chapters are proposed; they have not yet been accepted by TRB's Highway Capacity and Quality of Service (HCQS) Committee. The HCQS Committee has responsibility for approving the content of HCM2010.

SHRP 2 Reliability Project L08 has also released the FREEVAL and STREETVAL computational engines. The FREEVAL-RL computational engine employs a scenario generator that feeds the Freeway Highway Capacity Analysis methodology in order to generate a travel time distribution from which reliability metrics can be derived. The STREETVAL-RL computational engine employs a scenario generator that feeds the Urban Streets Highway Capacity Analysis methodology in order to generate a travel time distribution from which reliability metrics can be derived.

Software Disclaimer: This software is offered as is, without warranty or promise of support of any kind either expressed or implied. Under no circumstance will the National Academy of Sciences or the Transportation Research Board (collectively "TRB") be liable for any loss or damage caused by the installation or operation of this product. TRB makes no representation or warranty of any kind, expressed or implied, in fact or in law, including without limitation, the warranty of merchantability or the warranty of fitness for a particular purpose, and shall not in any case be liable for any consequential or special damages.

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