Design Guidance for Freeway Mainline Ramp Terminals (2012) / Chapter Skim
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Pages 33-102
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From page 33... ...
33 The objective of the observational study was to gather speed and distance information for a large number of merging and diverging vehicles covering a range of ramp types and merge types. From this data, speed profiles of vehicles could be developed to reflect current driver behavior and vehicle performance capabilities.
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From page 34... ...
34 • Vehicle mix (percent trucks) , and • Design speed of freeway.
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From page 35... ...
35 Ramp Ramp type Merge type Controlling feature Radius of controlling curve if ≤ 1,000 ft (ft) Grade Distance from painted nose to controlling feature (ft)
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From page 36... ...
36 straight ramps may also be limited by horizontal alignment if the ramp curves or bends. For this study, the horizontal alignment is only considered controlling if the radius of the curve is equal to or less than 1,000 ft, consistent with current guidance provided in the 2004 Green Book.
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From page 37... ...
37 grades ranging from a 6 percent downgrade to a 2 percent upgrade. Table 15 also provides for each entrance ramp the distance from the controlling feature to the painted nose, the length of the SCL from the painted nose to the beginning of the taper, and the length of the taper.
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From page 38... ...
38 similar at both entrance and exit ramps and for straight and loop ramps. Generally, field data were collected at each study site for 6 hours during a weekday during the following time periods: • AM or PM peak period -- 2 hours (i.e., the heavier directional peak period was chosen for data collection)
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From page 39... ...
39 were connected to laptop computers and their readings were automatically recorded in a separate spreadsheet for each vehicle at a rate of approximately three readings per second. The speed and distance data were plotted during post processing to develop speed profiles of individual vehicles along the ramps and acceleration or deceleration lanes.
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From page 40... ...
40 In addition to gathering data for the merging and diverging vehicles, traffic classifiers were used to record vehicle speeds and traffic volumes in the rightmost lane of the freeway at each study site. The traffic classifiers were positioned at three locations at each study site: • Entrance Ramps – Upstream of the painted nose: This classifier was positioned approximately 750 to 1,000 ft upstream of the painted nose.
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From page 41... ...
41 5.3.1 Laser Gun Data Processing The laser gun readings were processed using extensive quality checks, extrapolated as needed, and then smoothed using a local linear smoother. This was done to ensure that the profiles analyzed best reflected true vehicle profiles and that sources of measurement variability in the individual profiles were minimized.
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From page 42... ...
42 and for entrance ramps, the volumes were higher in the downstream. No major anomalies in the average speed between locations were observed.
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From page 43... ...
43 • A merging vehicle comes to a complete stop within or beyond the taper during free-merge conditions. • A following (i.e., platooned)
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From page 44... ...
44 (early) , in the middle third of the SCL (mid)
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From page 45... ...
45 merge conditions, a majority of vehicles merge at the early or mid positions, while for the constrained merge, a majority of vehicles merge in the late or taper positions. This observation is intuitive because under free-merge conditions, vehicles should have no trouble finding a gap and should be able to merge as soon as a comfortable merge speed is reached.
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From page 46... ...
46 Heavy truck Late/Taper 30.3% Passenger car Late/Taper 33.5% 11 Ramps 267 Heavy trucks 2,958 Passenger cars P er ce nt All Freeway Speeds Free Merge Constrained Merge Forced Merge Heavy truck Late/Taper 28.1% Passenger car Late/Taper 32.7% 11 Ramps 231 Heavy trucks 2,704 Passenger cars Heavy truck Late/Taper 66.7% Passenger car Late/Taper 56.0% 5 Ramps 9 Heavy trucks 100 Passenger cars Heavy truck Late/Taper 37.0% Passenger car Late/Taper 33.1% 5 Ramps 27 Heavy trucks 154 Passenger cars Vehicle type Heavy truck Passenger car 3 3 25 26 42 37 18 22 12 12 0 20 40 60 80 100 2 3 26 26 44 38 18 22 10 10 0 20 40 60 80 100 11 8 0 15 22 21 11 15 56 41 0 20 40 60 80 100 15 9 22 33 26 25 19 16 19 17 0 20 40 60 80 100 Gore Early Mid Late Taper Figure 16. Merge location -- all ramps by vehicle type (passenger cars versus trucks)
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From page 47... ...
47 Loop Late/Taper 16.0% Straight Late/Taper 36.9% 567 Vehicles on loop ramps 2,658 Vehicles on straight ramps P er ce nt All Freeway Speeds Free Merge Constrained Merge Forced Merge Loop Late/Taper 16.0% Straight Late/Taper 36.3% 567 Vehicles on loop ramps 2,368 Vehicles on straight ramps Loop Late/Taper 0% Straight Late/Taper 56.9% 0 Vehicles on loop ramps 109 Vehicles on straight ramps Loop Late/Taper 0% Straight Late/Taper 33.7% 0 Vehicles on loop ramps 181 Vehicles on straight ramps Type of Ramp Loop (2 ramps) Straight (9 ramps)
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From page 48... ...
48 Parallel Late/Taper 24.7% Taper Late/Taper 52.5% 2,231 Vehicles on parallel ramps 994 Vehicles on taper ramps P er ce nt All Freeway Speeds Free Merge Constrained Merge Forced Merge Parallel Late/Taper 24.2% Taper Late/Taper 51.0% 2,043 Vehicles on parallel ramps 892 Vehicles on taper ramps Parallel Late/Taper 31.0% Taper Late/Taper 73.1% 42 Vehicles on parallel ramps 67 Vehicles on taper ramps Parallel Late/Taper 29.5% Taper Late/Taper 51.4% 146 Vehicles on parallel ramps 35 Vehicles on taper ramps Type of Merge Parallel (7 ramps) Taper (4 ramps)
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From page 49... ...
49 No Late/Taper 36.2% Yes Late/Taper 24.0% 2,461 Vehicles on ramps not meeting criteria 764 Vehicles on ramps meeting criteria P er ce nt All Freeway Speeds Free Merge Constrained Merge Forced Merge No Late/Taper 35.1% Yes Late/Taper 24.2% 2,199 Vehicles on ramps not meeting criteria 736 Vehicles on ramps meeting criteria No Late/Taper 66.7% Yes Late/Taper 10.5% 90 Vehicles on ramps not meeting criteria 19 Vehicles on ramps meeting criteria No Late/Taper 33.7% Yes Late/Taper 33.3% 172 Vehicles on ramps not meeting criteria 9 Vehicles on ramps meeting criteria Ramp Meets Criteria? No (8 ramps)
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From page 50... ...
50 Free-flow Late/Taper 33.2% Platooned Late/Taper 33.4% 11 Ramps 2,102 Free-flow vehicles 1,123 Platooned vehicles P er ce nt All Freeway Speeds Free Merge Constrained Merge Forced Merge Free-flow Late/Taper 32.9% Platooned Late/Taper 31.3% 11 Ramps 2,005 Free-flow vehicles 930 Platooned vehicles Free-flow Late/Taper 31.3% Platooned Late/Taper 67.5% 5 Ramps 32 Free-flow vehicles 77 Platooned vehicles Free-flow Late/Taper 44.6% Platooned Late/Taper 27.6% 5 Ramps 65 Free-flow vehicles 116 Platooned vehicles Vehicle position Free-flow Platooned 3 3 25 28 38 35 22 20 11 13 0 20 40 60 80 100 3 2 26 28 39 38 22 22 11 9 0 20 40 60 80 100 16 5 16 13 38 14 28 9 3 58 0 20 40 60 80 100 6 12 17 40 32 21 23 13 22 15 0 20 40 60 80 100 Gore Early Mid Late Taper Figure 20. Merge location -- by vehicle position (free-flow versus platooned)
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From page 51... ...
51 taking the first available gap, while in forced-merge conditions, the slower freeway speeds may allow platooned vehicles to merge into freeway traffic at an earlier location than the lead vehicle. It should also be noted that under free-merge conditions, when about twice as many free-flow vehicles as platooned vehicles were observed while under constrained- and forced-merge conditions, the ratio was reversed, with nearly twice as many observations of platooned vehicles as free-flow vehicles merging.
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From page 52... ...
52 conditions, in which case a larger percentage of platooned vehicles merge later than free-flow vehicles. 5.4.1.2 Merge Speed The data presented in this section compare the speed of merging vehicles at the time they begin to merge to the speed of freeway traffic in the rightmost lane.
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From page 53... ...
53 the available combinations of ramp type, merge type, vehicle type, and merge condition. For each ramp and merge type, the mean value is the average of the individual ramp averages.
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From page 54... ...
54 Ramps meet current criteria Type of merge No. of ramps Vehicle type N obs Mean Std dev Min Max No Parallel 5 Trucks 198 -22.0 8.1 -55.6 11.5 Passenger cars 1,522 -16.1 8.0 -52.0 21.2 Taper 3 Trucks 40 -10.0 6.4 -20.0 9.1 Passenger cars 701 -5.7 8.7 -53.5 16.9 Yes Parallel 2 Trucks 27 -10.4 8.2 -33.0 3.8 Passenger cars 484 -5.1 6.3 -34.4 23.8 Taper 1 Trucks 2 -10.7 2.7 -12.7 -8.8 Passenger cars 251 -7.6 9.5 -50.3 13.5 No Both merge types combined 8 Trucks 238 -17.5 7.9 -55.6 11.5 Passenger cars 2,223 -12.2 8.2 -53.5 21.2 Yes Both merge types combined 3 Trucks 29 -10.5 8.1 -33.0 3.8 Passenger cars 735 -6.0 7.6 -50.3 23.8 Table 21.
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From page 55... ...
Figure 23. Merge speed differential by ramp type, merge type, vehicle type, and merge location (ramp type: loop; merge type: parallel)
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From page 56... ...
Figure 25. Merge speed differential by ramp type, merge type, vehicle type, and merge location (ramp type: straight; merge type: parallel)
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From page 57... ...
57 observations were only made during free-merge conditions. Trucks have a greater speed differential than passenger cars, and early mergers have a greater speed differential than later mergers.
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58 • Passenger cars merge at speeds closer to freeway speeds than do trucks. This is expected as trucks do not accelerate as quickly as passenger cars and would be unlikely to reach the same speeds as cars by the merge location.
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From page 59... ...
Figure 28. Acceleration profiles and cumulative merge curve by merge condition (NB entrance ramp at I-435/US 24 and WB entrance ramp at I-376/Wm.
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From page 60... ...
Figure 30. Acceleration profiles and cumulative merge curve by merge condition (NB entrance ramp at I-435/Gregory Blvd and WB entrance ramp at I-635/Midway Rd)
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From page 61... ...
61 right-side y-axis. As more vehicles merge, fewer observations are included in the boxes.
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From page 62... ...
Ramp location Diff. between actual accel.
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From page 63... ...
Ramp location Diff. between actual accel.
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From page 64... ...
Ramp location Diff. between actual accel.
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From page 65... ...
Ramp location Diff. between actual accel.
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From page 66... ...
66 lower section. Within the upper section, the first two ramps are loop ramps (where the controlling feature -- horizontal curvature -- ends at the painted nose)
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From page 67... ...
67 to Table 23, platooned passenger cars are traveling slightly slower than free-flow passenger cars at the location of initial speed measurement, most likely because their speed is limited by the vehicles they are following. Table 25 shows that the initial speed measured for free-flow trucks is around 5 mi/h less than that measured for free-flow passenger cars in Table 22, which indicates that trucks travel at slower speeds around the controlling horizontal curve and accelerate at a lower rate from the crossroad terminal.
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From page 68... ...
68 Design speed (mi/h) Speed reached (mi/h)
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From page 69... ...
69 not include deceleration were included in the analysis. For example, if a vehicle's initial speed was 43 mi/h, but then fell to 41 mi/h, this section of the speed profile was disregarded.
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From page 70... ...
70 Design speed (mi/h) Speed reached (mi/h)
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From page 71... ...
71 passenger cars. Thus, even though the grade adjustment factors based on acceleration capabilities of trucks are comparable to values in the Green Book, this does not necessarily mean that by applying the grade adjustment factors for upgrades that the corresponding minimum acceleration lane lengths would accommodate the vehicle performance capabilities of trucks.
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From page 72... ...
72 as measured in the field. Then a weight-to-power ratio was input and adjusted to obtain a profile that matched the speed profile measured in the field as closely as possible.
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From page 73... ...
73 • Merging vehicles sometimes reach a peak speed along the ramp and/or SCL and decelerate to slower speeds prior to merging onto the freeway. This behavior is most evident under constrained and forced-merge conditions, when freeway speeds are reduced.
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From page 74... ...
74 Vehicle Speed Profile From Painted Nose to Merge Completion 20 25 30 35 40 45 50 0 50 100 150 200 250 300 350 400 S pe ed (m ph ) Distance From Painted Nose (ft)
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From page 75... ...
0 20 40 60 80 100 120 140 160 180 200 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 W ei gh t/P ow er R at io (lb /hp ) Truck # Weight/HP ratio (all trucks: free-flow)
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From page 76... ...
76 Acceleration on Upgrades and Downgrades 0 10 20 30 40 50 60 0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 Sp ee d (m ph ) Distance (ft)
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From page 77... ...
77 Acceleration on Upgrades and Downgrades Deceleration on Upgrades 9% 1% 2% 3% 4% 5% 6% 7% 8% 0% 0 10 20 30 40 50 60 70 80 0 5,000 10,000 15,000 20,000 25,000 Sp ee d (m ph ) Distance (ft)
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From page 78... ...
78 Acceleration on Upgrades and Downgrades Deceleration on Upgrades 8% -5% -4% -3% -2% -1% 0% 1% 2% 3% 4% 5% 6% 7% 0 10 20 30 40 50 60 0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 Sp ee d (m ph ) Distance (ft)
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From page 79... ...
79 -5% -4 % -3% -2% -1% 0% 1% 2% 3% 4% 5% 6% 7% 8% 0 10 20 30 40 50 60 0 2 ,0 00 4, 000 6, 000 8, 00 0 1 0, 000 12, 000 14, 000 16 ,0 00 Sp ee d (m ph )
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From page 80... ...
80 Four instances were observed in which a following (i.e., platooned) vehicle in the SCL passed the leading vehicle in the SCL using the right shoulder.
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From page 81... ...
81 Figure 42. Diverge location -- all ramps by vehicle type.
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From page 82... ...
82 uncongested ramp, but the lower initial speed at the diverge point allows them to exit the freeway at a more casual deceleration rate. It is also important to note that because such a high proportion of the observed vehicles exited under free-diverge conditions within the taper, analysis of the entire population of observed vehicles is heavily influenced by this block.
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From page 83... ...
83 Figure 43. Diverge location -- by ramp type.
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From page 84... ...
84 Figure 44. Diverge location -- by diverge type.
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From page 85... ...
85 Figure 45. Diverge location -- by vehicle position.
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From page 86... ...
86 individual ramp averages. Also provided are the standard deviation (pooled across individual ramps)
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From page 87... ...
87 free-diverge conditions, the following results were obtained from the ANOVA model: • The between-ramp standard deviation in the mean diverge speed differential was estimated to be 1.09 mi/h. The within-ramp (vehicle-to-vehicle)
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From page 88... ...
88 Figure 47. Diverge speed differential by ramp type, diverge type, vehicle type, and diverge location (ramp type: loop; diverge type: parallel)
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From page 89... ...
89 Figure 49. Diverge speed differential by ramp type, diverge type, vehicle type, and diverge location (ramp type: straight; diverge type: parallel)
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From page 90... ...
90 slightly greater than average freeway speeds for both passenger cars and trucks. Figure 48 presents observations made at loop ramps with tapered SCLs.
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From page 91... ...
91 deceleration lane length. Section 3 of this report provides two methods for back-calculating deceleration rates based on Green Book Exhibit 10-73.
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From page 92... ...
92 1,000 ft, consistent with current guidance provided in the 2004 Green Book. Average Speed at Green Book Distance -- The average speed recorded for observed vehicles at the distance corresponding to the Green Book deceleration length: the average observed speed at the location where the vehicle has traveled the minimum Green Book distance from the point of diverge.
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From page 93... ...
Ramp location GB initial speed (mi/h) Average diverge speed (mi/h)
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From page 94... ...
Ramp location GB initial speed (mi/h) Average diverge speed (mi/h)
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From page 95... ...
Ramp location GB initial speed (mi/h) Average diverge speed (mi/h)
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From page 96... ...
96 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0 100 200 300 400 500 600 700 800 900 1000 D ec el er at io n Ra te (f t/s 2 ) Length of Ramp Greater than Green Book Length (ft)
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From page 97... ...
97 Comparison of Deceleration Rates: Deceleration Rates from Green Book Criteria for Level Grades Assuming a Constant Deceleration. Table 34 provides a comparison of the deceleration rates derived from the Green Book recommended deceleration lane lengths for level grades to deceleration rates measured in the field for various vehicle groups under various conditions.
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From page 98... ...
98 Condition 3 -- This category considers all passenger cars (i.e., both free-flow and platooned) that are exiting the freeway under constrained-diverge conditions, when the freeway speed is between 40 and 50 mi/h.
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From page 99... ...
99 Table 37 shows the distribution of deceleration rate by vehicle type. There was not a great deal of difference between rates for passenger cars and rates for heavy trucks overall (–2.72 and –2.82 ft/s2, respectively)
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From page 100... ...
100 Ramp type Number of vehicles Average deceleration rate (ft/s2) Loop 1,325 –2.99 Straight 939 –2.35 Total 2,264 –2.73 Table 38.
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From page 101... ...
101 Comparison of Deceleration Rates on Grades of 3 Percent or Greater. Two of the exit ramp locations had grades that could not be considered in the level-grade category with the other nine ramps: I-435 at Gregory (3 percent downgrade)
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From page 102... ...
102 • There was very little difference between deceleration rates for passenger cars and heavy trucks overall; however, comparison of rates for a given initial speed suggest that trucks decelerated at a greater rate than cars. • There was a noticeable difference between deceleration rates on loop ramps and straight ramps, with loop ramps having greater deceleration.
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