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Pages 81-98

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From page 81...
... 81 S e c t i o n 8 Section 8 focuses on development of improved methodologies for predicting the speeds of trucks on freeways under varying vertical grade conditions. 8.1 Existing HCM Treatment of Trucks on Freeways The current HCM freeway procedures are not designed to predict truck speeds on freeways.
From page 82...
... 82 incorporating truck Analysis into the Highway capacity Manual Passenger Car Equivalent Level Terrain Rolling Terrain Mountainous Terrain ET (trucks and buses)
From page 83...
... Prediction of Freeway Truck Speeds 83 2. Select a test site with a calibrated microsimulation model for investigating the relationship between volumes, truck percentages, the mix of truck types, and vertical grade on sustained automobile and truck speeds.
From page 84...
... 84 Incorporating Truck Analysis into the Highway Capacity Manual Exhibit 55 shows a plot of the 15-min observations of flow and density. It is obvious that the truck percentage does have an effect on both the maximum flow rate that is achieved and the density that corresponds to that maximum flow rate.
From page 85...
... Prediction of Freeway Truck Speeds 85 8.4 The Freeway Test Bed The investigation of the freeway speed flow effects of trucks on extended grades was performed on a hypothetical facility consisting of 8 miles of level three-lane freeway followed by 5 miles of three-lane freeway with a grade. The grade on the 5-mile section varied from –6% to +6% in 1% increments (13 grades total)
From page 86...
... 86 Incorporating Truck Analysis into the Highway Capacity Manual curve is found in the paper "Survey of Uphill Speeds of Trucks on Mountain Grades" (Willey, 1949)
From page 87...
... Prediction of Freeway truck Speeds 87 8.5.2 Approach to Calibrating VISSIM Truck Profiles Steps 2 and 3 started with a review of the literature on truck performance. The review showed that the procedure described in NCHRP Report 505 (Harwood et al., 2003)
From page 88...
... 88 incorporating truck Analysis into the Highway capacity Manual After making these changes, the new model's predictions were compared with those from a model developed by Rakha et al.
From page 89...
... Prediction of Freeway truck Speeds 89 8.8 The Speed Prediction Models The steps in the procedure that focus on predicting the truck speed are motivated by Exhibit 59. It shows the speed-flow plot for this same condition.
From page 90...
... 90 incorporating truck Analysis into the Highway capacity Manual Predicting the truck speeds proved to be relatively simple. In all 636 scenarios involving trucks (including the 100% truck case)
From page 91...
... Prediction of Freeway truck Speeds 91 WtFa = the weight-to-frontal-area ratio (lb/ft2) ; aele = an altitude-related adjustment factor for air resistance for converting sea-level aerodynamic drag to local elevation is equal to (1 – 0.000006887 ∗ ft.
From page 92...
... 92 incorporating truck Analysis into the Highway capacity Manual A simple logistics function is used to predict these automobile speed trends: 1 Equation 39s s s s e e a to ao to v v V v v V m m ( ) = + − +       −β − ∆ −β − ∆ where sa = the automobile speed at flow rate v; sao = the automobile-only speed that would arise at flow rate v (taking into account the PCE value)
From page 93...
... Prediction of Freeway truck Speeds 93 Grade >= 1%? Grade < 4% AND TrkType-9 AND Wt/Hp=50 %Trks >= 30% Yes No Yes vm = 150 vm = 1000 - 0.3475∗%Trk - (1.226 + 0.07∗%Trk)
From page 94...
... 94 incorporating truck Analysis into the Highway capacity Manual The t statistic, shown below the coefficient for vm, being significantly greater than 1.97, demonstrates that the effect is statistically significant. In summary, the prediction procedure for truck speed is relatively simple.
From page 95...
... Prediction of Freeway truck Speeds 95 � –0.80% for 1,192 ft.; and � –2.10% for 5,299 ft. • Eastbound: opposite grades for same lengths.
From page 96...
... 96 incorporating truck Analysis into the Highway capacity Manual As can be seen from Exhibit 65, the proposed analysis procedure ("New HCM") predicts auto mobile and truck speeds that are generally consistent (within 6%)
From page 97...
... Prediction of Freeway truck Speeds 97 classes (labeled "passenger vehicles") ; and all vehicles combined.
From page 98...
... 98 incorporating truck Analysis into the Highway capacity Manual The median (50th-) and 95th-percentile TTIs predicted using the SHRP2-L08 method are entered into the following two equations, which are solved for the values of the parameters k and c: 50% 2 Equation 43 1 1TTI kc( )

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