The present chapter contains the findings of this research effort. General database information is presented first, in order to establish relationships between the fundamental traffic flow parameters at the study location (volume, speed, density, free flow speed, peaking characteristics, traffic mix, and number of trucks throughout the day). This is accomplished through general descriptive statistics (frequencies and means) tables and graphs (bar charts, histograms).
Subsequently the analysis focus shifts to testing a number of hypotheses set to examine Heavy Vehicle (HV) influence on freeway traffic operations. A major part of this effort is focused on headways of individual and lagging-leading pairs of vehicles, and the relationship of this variable with other traffic flow parameters, such as speed, volume, truck presence (percent of trucks) and vehicle weight.
5.2 General Study Site Traffic Flow Characteristics
A total of 1,201,053 vehicles were observed during the study period, of which 84.8% were passenger cars (PC) and 9.0% were light trucks. Among heavy vehicles, single-trailer trucks with five axles (vehicle class 9-see Figure 3.1) were the most prevalent at 1.7% and single-trailer trucks with four or less axles (vehicle class 8-see Figure 3.1) were 1.6% of the total vehicle counts. Table 5.1 shows the frequency of each vehicle type in the total vehicle observations.
was 70,675 vehicles per day in all three lanes together. Weekday peak hours of traffic were observed generally between 6 am to 9 am and again from 3 pm to 6 pm. During weekends, peak traffic was observed between 11 am and 7 pm. Figures 5.1 and 5.2 stacked bar histograms present weekday and weekend hourly traffic patterns with light vehicles (classes 2 and 3) shown in blue and heavy vehicles (classes 4 and above) shown in green.
TABLE 5.1 Frequencies of vehicles observed at the study site.
FHWA
class
Frequency Percent Valid
Percent
Cumulative Percent
Motorcycle 1 3,427 .3 .3 .3
PC 2 1,018,981 84.8 84.8 85.1
Four-tire single-unit truck 3 108,584 9.0 9.0 94.2
Buses 4 9,011 .8 .8 94.9
Two-axle, six-tire single-unit truck 5 13,953 1.2 1.2 96.1
Three-axle, single-unit truck 6 6,187 .5 .5 96.6
Four-or-more- axle, single-unit truck 7 290 .0 .0 96.6
Four-or-less axle, single-unit trailer 8 19,694 1.6 1.6 98.3
Five-axle, single trailer 9 20,467 1.7 1.7 100.0
Six-or-more-axle, single trailer 10 91 .0 .0 100.0
Five-or-less-axle, multi-trailer 11 301 .0 .0 100.0
Six-axle, multi-trailer 12 67 .0 .0 100.0
Seven-or-more-axle, multi-trailer 13 2 .0 .0 100.0
29
30
5.3 Free Flow Speed
According to the HCM 2000, Free-Flow Speed (FFS) is the mean speed of passenger cars measured during low to moderate flows (up to 1,300 pc/h/ln). This average PC speed reflects the net effect of all prevailing geometric conditions, such as lane width, lateral clearance, interchange density, number of lanes, speed limit and vertical and horizontal alignment that influence speed. The HCM 2000 indicates that speed data that include PC and HV can be used to determine FFS for level terrain or moderate downgrades but should not be used for rolling or mountainous terrain (2). As the data for this research was collected from a level freeway segment, the FFS was determined for the mixed traffic stream containing both PC and HV. Figure 5.3 shows the Speed-Volume curve developed for the study site.
32
The Speed-Volume curve was developed using hourly per-lane volumes based on average 15-minute flows and the corresponding 15-min average speeds of the mixed traffic stream.
The ―rays‖ (straight lines converging at the origin) of Figure 5.3 indicate the boundaries between Levels of Service (LOS), segregating the collected data into distinct ―sectors‖ for each LOS. Visual inspection reveals that, as congestion levels increase (moving from left-to-right) in Figure 5.3, average speeds start decreasing within LOS C at about 1,200 vphpl.
In order to adhere to the HCM FFS definition intent (the average speed measured at low to moderate flows), speed observations corresponding to flows less than or equal to 1200 vphpl were selected and their average value (61 mph) was considered to be the study location‘s FFS (see horizontal line at 61 mph).
The capacity of the study location (based on the highest 15-minute measured flows) was determined to be approximately 2,500 vphpl.
5.4 Headway Relation with Speed
To examine the vehicle headway relation with speed, individual vehicle speeds were divided into 10 speed ranges: 0-10 mph, 10-15 mph, 15-20 mph, 20-25 mph, 25-30 mph, 30-35 mph, 35-40 mph, 40-45 mph, 45-50 mph and greater than 50 mph. The mean headway for each of these speed ranges was calculated. A strong relationship between mean vehicle headway and vehicle speed was identified (r = 0.981 – see Table 5.2). It was found that headway decreased with an increase in vehicle speed and reached its minimum value at a range of 40-45 mph; above 45 mph, headway started increasing with increasing speed. Thus it was documented that the analyzed freeway section reached its capacity (coincident with minimum headways) within the range of 40-45 mph. Figure 5.4 shows average headway relationship with speed.
A regression analysis using average headway as the dependent and speed range as the independent variable had an excellent fit at the 0.000 level of significance (Table 5.3) when a Quadratic equation was fit to the data (see coefficients on Table 5.4 – all regression model coefficients were significant at the 0.000 level). The relation can be expressed by the following equation:
𝑉𝑒ℎ𝑖𝑐𝑙𝑒 𝐻𝑒𝑎𝑑𝑤𝑎𝑦 = 0.0018 ∗ 𝑆𝑝𝑒𝑒𝑑2+ −0.14 ∗ 𝑆𝑝𝑒𝑒𝑑 + 4.94 5.1
Where, Vehicle Headway was measured in seconds.
35
TABLE 5.2 Regression model summary of Headway vs. Vehicle speed.
R R Square Adjusted R Square Std. Error of the Estimate
.981 .962 .951 .149
TABLE 5.3 ANOVA results for Headway vs. Vehicle Speed regression analysis.
Sum of Squares df Mean Square F Sig.
Regression 3.947 2 1.973 88.787 .000
Residual .156 7 .022
Total 4.102 9
TABLE 5.4 Coefficient values for the Headway vs. Vehicle Speed equation.
Unstandardized Coefficients Standardized
Coefficients t Sig.
B Std. Error Beta
Speed -.140 .015 -3.152 -9.443 .000
Speed ** 2 0.0018 .000 2.383 7.137 .000
5.5 Heavy Vehicle Effect on Lighter Vehicles
A total of 1,200,992 lag-lead vehicle pair observations were available from field data. Enough observations for analysis purposes were available for vehicle pairs involving PC (FHWA class 2), four-tire single-unit trucks (FHWA class 3), four-or-less-axle single-trailer trucks (FHWA class 8) and five-axle single-trailer trucks (FHWA class 9).
A primary hypothesis of this research was that the presence of HV in mixed traffic stream had a negative impact on freeway capacity. This hypothesis was addressed in the present effort in a number of ways, focusing primarily on the effects HV had on headways.
5.5.1 Lagging PC Headways
Previous research efforts determined differences in the headways maintained between specific pairs of vehicle classes. Typically, lighter vehicles were found to maintain larger headways when following (lagging) heavier vehicles than when following other lighter vehicles (4). This finding was tested herein with the available field data. For this effort, lighter vehicles in the database were paired with their leading vehicles. It was desired to test the hypothesis that lighter vehicles would keep longer headways when following a heavier vehicle, than when following another lighter vehicle.
Among the available light vehicle-following-light vehicle pairs, ―PC- following-PC‖ (FHWA vehicle class 2) and ―PC-following-four-tire single-unit truck‖ (FHWA Truck class 3) had a significant presence in the traffic stream.
Among light vehicle-following-heavier vehicle pairs, only ―PC-following-four-or- less-axle, single-trailer truck‖ (FHWA Truck class 8) and ―PC-following-five axle, single-trailer truck‖ (FHWA Truck class 9) pairs had significant numbers of observations for analysis. Table 5.5 shows major lagging-leading vehicle pairs with a lagging PC and the corresponding frequencies.
It was shown earlier that a strong relationship exists between average headway and speed. It was desired to investigate whether headway-speed relationships between particular vehicle class pairs followed this general trend, and whether statistically significant differences existed in headways maintained between particular vehicle classes. The same speed ranges used in section 5.4 to investigate the headway-speed relationship, 0-10 mph, 10-15 mph, 15-20 mph, 20-25 mph, 25-30 mph, 30-35 mph, 35-40 mph, 40-45 mph, 45-50 mph and greater than 50 mph, were used in the present lagging-leading vehicle pair headway investigation, as well. Due to an absence of sufficient observations for PC following trucks class 8 or 9, the speed range 0-10 mph was excluded from the analysis. Only headways equal to or shorter than 10 seconds were included since longer headways are not typically due to vehicle interactions, but, rather, random arrivals under very low volume conditions. Also for speed range 10-15 mph, PC-following-Truck class 9 had only 28 observations. This number was insufficient to provide a representative result for actual field conditions.
An analysis of Variance (ANOVA) of headway (dependent variable) versus Lagging-Leading pair (fixed factor) for each of the ten analyzed speed ranges was used in this investigation. Descriptive statistics, statistical significance and
95% Confidence Intervals (95% CI) for average headway values are shown in Tables 5.6 and 5.7. The 95% headway CI for each vehicle pair type at each analyzed speed range is presented graphically in Figure 5.5.
Table 5.6 shows general headway descriptive statistics for headways maintained between particular vehicle pairs; for example, the average headways between two PC for the speed range of 15-20 mph was 2.65 seconds. The 95% CI for that value was 2.62-2.68 seconds. The average headway for a PC following a truck class 9 within the same speed range was 4.61 seconds with a 95% CI 4.39 to 4.82 seconds. Since the two 95% CI for these two types of Lagging-Leading vehicle pairs did not overlap, the average headway lagging PC maintained from leading trucks class 9 was statistically significantly larger than the average headway they maintained from leading PC at the 95% confidence level.
This same comparison is shown in Table 5.7 for these two vehicle type pairs. In this case headway differences between a Lagging-Leading PC-PC (column (I) Lagging-Leading vehicle pair) and a Lagging-Leading PC-truck class 9 pair (column (J) Lagging-Leading vehicle pair) is shown in column Mean Difference (I-J). In the above example, for the 15-20 mph speed range, the value -1.95 seconds corresponds to the difference 2.65-4.61 seconds (from Table 5.6). The 95% CI for this difference is -2.24 to -1.66 seconds; the difference is statistically significant at the 0.000 level of significance (less than 1/10,000 probability that no difference exists between the two headway populations). Since the 95% CI does not include zero (which would indicate that the two
average headways were equal), it can be stated with a high degree of certainty that PC-PC headways are on average 1.95 seconds lower than PC-truck class 9 headways at this speed range.
Within each speed range, the average headways maintained by lagging PC were shortest from leading PC, and progressively increased from leading small trucks class 3, trucks class 8 and trucks class 9.
Findings for individual vehicle classes were consistent with the overall headway-speed observations. Headways decreased with increasing speed with the shortest headways in the 40-45 mph speed range at 1.96 for PC-following- PC, 2.00 seconds for PC-following-small trucks class 3 and 2.35 seconds for PC- following-trucks class 8. The shortest headways for PC-following-trucks class 9 were observed at the 45-50 mph speed range. Headways started increasing as speed kept increasing past the corresponding minimum headway speed ranges.
The level of significance (Sig.) and mean difference (I-J) columns on Table 5.7 in conjunction with the graphical representation of the 95% CI of mean headways in Figure 5.5 lead to the following observations about lagging PC headways:
I. Headways from PC were shorter than headways from small trucks (class 3), however differences were no more than 0.15 seconds at any speed range;
II. Headway differences from PC and small trucks class 3 were not statistically significantly different among themselves (level of significance exceeds 0.141 in most cases but even where statistical significance
exists, differences were small and had negligible practical consequences); III. Headways from heavier vehicles (trucks class 8 and trucks class 9) were
statistically significantly higher for all speed ranges. (The only exception was class 8 in the 20-25 mph speed range where 95% PC, truck class 3 and truck class 8 CI overlap);
IV. Headway differences between PC and small trucks class 3 on one hand, and heavier trucks classes 8, 9 on the other, were the longest at the lowest speeds. Headway differences from trucks class 9 decreased with speed; for trucks class 8 the pattern was not consistent with speed.
TABLE 5.5 Types of Lagging-Leading pairs with lagging Passenger Cars (PC) and their
frequencies in the dataset. Lagging-Leading vehicle pair
No. of observations Percent Cumulative Percent Light vehicle-following- Light vehicle PC-PC 872,100 87.8 87.8
PC-Small truck class 3 90,113 9.1 96.9
Light vehicle-following- Heavy vehicle
PC-Truck class 8 15,375 1.5 98.5
PC-Truck class 9 15,339 1.5 100.0
TABLE 5.6 Headway-Speed relationships for vehicle pairs with Lagging Passenger Cars (PC). Lagging Vehicle speed range (mph) Lagging-Leading vehicle pair N Mean Std. Deviation Std. Error 95% Confidence Interval for Mean
Lower Bound Upper Bound 10-15 PC-PC 2,720 3.18 1.35 .03 3.13 3.23
PC- Small truck class 3 445 3.33 1.45 .07 3.19 3.46
PC-Truck class 8 93 4.14 1.55 .16 3.82 4.46
PC-Truck class 9 28 5.51 1.84 .35 4.80 6.22
15-20
PC-PC 6,443 2.65 1.24 .02 2.62 2.68
PC-Small truck class 3 1,051 2.78 1.29 .04 2.70 2.86
PC-Truck class 8 315 2.97 1.71 .10 2.78 3.16
PC-Truck class 9 131 4.61 1.25 .11 4.39 4.82
20-25
PC-PC 7,557 2.43 1.20 .01 2.40 2.46
PC-Small truck class 3 1,106 2.50 1.24 .04 2.42 2.57
PC-Truck class 8 262 2.54 1.45 .09 2.36 2.72
PC-Truck class 9 161 4.01 1.25 .10 3.81 4.21
25-30
PC-PC 7,470 2.35 1.28 .01 2.32 2.38
PC-Small truck class 3 1,037 2.38 1.31 .04 2.30 2.46
PC-Truck class 8 222 2.79 1.46 .10 2.60 2.98
PC-Truck class 9 145 3.60 1.43 .12 3.37 3.84
30-35
PC-PC 8,324 2.29 1.33 .01 2.26 2.31
PC-Small truck class 3 1,067 2.33 1.33 .04 2.25 2.41
PC-Truck class 8 206 2.52 1.38 .10 2.33 2.71
PC-Truck class 9 141 3.12 1.12 .09 2.93 3.30
35-40
PC-PC 10,253 2.18 1.39 .01 2.15 2.21
PC-Small truck class 3 1,121 2.26 1.39 .04 2.17 2.34
PC-Truck class 8 193 2.62 1.47 .11 2.41 2.83
PC-Truck class 9 143 2.93 1.29 .11 2.71 3.14
40-45
PC-PC 17,516 1.96 1.33 .01 1.94 1.98
PC-Small truck class 3 1,725 2.00 1.38 .03 1.94 2.07
PC-Truck class 8 322 2.35 1.38 .08 2.20 2.50
PC-Truck class 9 230 2.77 1.27 .08 2.60 2.93
45-50
PC-PC 45,633 2.06 1.66 .01 2.05 2.08
PC-Small truck class 3 4,562 2.03 1.63 .02 1.98 2.07
PC-Truck class 8 919 2.49 1.64 .05 2.38 2.59
PC-Truck class 9 785 2.62 1.55 .06 2.51 2.73
>50
PC-PC 691,379 2.60 2.06 .00 2.59 2.60
PC-Small truck class 3 70,715 2.56 2.05 .01 2.54 2.57
PC-Truck class 8 11,279 2.87 2.01 .02 2.83 2.91
43
TABLE 5.7 Types of Lagging-Leading vehicle pairs with Lagging Passenger Cars (PC).
Lagging Vehicle speed range(mph)
(I) Lagging- Leading vehicle
pair
(J) Lag-Leading vehicle pair
Mean Difference (I-J) Std. Error Sig. 95% Confidence Interval Lower Bound Upper Bound
10-15 PC-PC PC-Small truck class 3 -0.15 0.07 0.141 -0.33 0.03
PC-Truck class 8 -0.96 0.14 0.000 -1.33 -0.59
PC-Truck class 9 -2.33 0.26 0.000 -3.00 -1.66
15-20 PC-PC PC-Small truck class 3 -0.13 0.04 0.014 -0.24 -0.02
PC-Truck class 8 -0.32 0.07 0.000 -0.50 -0.13
PC-Truck class 9 -1.95 0.11 0.000 -2.24 -1.66
20-25 PC-PC PC-Small truck class 3 -0.07 0.04 0.301 -0.17 0.03
PC-Truck class 8 -0.11 0.08 0.454 -0.31 0.08
PC-Truck class 9 -1.58 0.10 0.000 -1.83 -1.33
25-30 PC-PC PC-Small truck class 3 -0.03 0.04 0.933 -0.14 0.08
PC-Truck class 8 -0.44 0.09 0.000 -0.66 -0.21
PC-Truck class 9 -1.25 0.11 0.000 -1.53 -0.98
30-35 PC-PC PC-Small truck class 3 -0.05 0.04 0.674 -0.16 0.06
PC-Truck class 8 -0.23 0.09 0.060 -0.47 0.01
PC-Truck class 9 -0.83 0.11 0.000 -1.12 -0.54
35-40 PC-PC PC-Small truck class 3 -0.08 0.04 0.286 -0.19 0.03
PC-Truck class 8 -0.44 0.10 0.000 -0.70 -0.18
PC-Truck class 9 -0.75 0.12 0.000 -1.05 -0.45
40-45 PC-PC PC-Small truck class 3 -0.05 0.03 0.492 -0.13 0.04
PC-Truck class 8 -0.39 0.07 0.000 -0.59 -0.20
PC-Truck class 9 -0.81 0.09 0.000 -1.04 -0.58
45-50 PC-PC PC-Small truck class 3 0.03 0.03 0.554 -0.03 0.10
PC-Truck class 8 -0.43 0.06 0.000 -0.57 -0.29
PC-Truck class 9 -0.56 0.06 0.000 -0.71 -0.41
>50 PC-PC PC-Small truck class 3 0.04 0.01 0.001 0.01 0.06
PC-Truck class 8 -0.26 0.02 0.000 -0.32 -0.21
44
FIGURE 5.5 95% mean Headway Confidence Intervals for Lagging-Leading vehicle pairs with Lagging Passenger Cars (PC) within specific speed ranges.
5.5.2 Passenger car-Heavy Vehicle Pair Headways
Although headway differences were identified in the literature depending on whether a PC leads or lags a HV, there was no agreement among researchers about the direction of these differences. HV following (lagging) PC was found to maintain larger headways than PC following HV (6, 10). However, another study found that HV traveled closer to PC than PC following HV (9).
Given the ambiguity of previous findings, it was desired to provide answers based on the study database. Furthermore, given the already documented strong headway-speed relationship, it was desired to identify any statistically significant headway differences and quantify their magnitudes within each of the previous defined speed ranges.
Headway differences depending on whether a PC leads or lags a HV were tested for two HV classes, trucks class 8 and class 9. Two separate hypotheses were examined. The first tested for statistically significant headway differences between PC leading or lagging a truck class 8; the second examined a similar hypothesis for trucks class 9.
The mean headway for all examined vehicle combinations was analyzed for the following nine vehicle speed ranges: 10-15 mph, 15-20 mph, 20-25 mph, 25-30 mph, 30-35 mph, 35-40 mph, 40-45 mph, 45-50 mph and greater than 50 mph. Not enough data were available for the 0-10 mph speed range; headways greater than 10 seconds were excluded because they were not due to vehicle interactions, but, rather, random arrivals under very low volume conditions.
Lagging-Leading pair (fixed factor) for each of the nine analyzed speed ranges was used to address each of the two hypotheses. Headway descriptive statistics (number of cases, mean, standard deviations and standard error) as well as 95% Confidence Intervals (95% CI) for means are provided on Table 5.8. The 95% CI for headway means involving trucks class 8 are graphically depicted in Figure 5.6; Figure 5.7 shows similar information for trucks class 9.
The numeric values of the upper and lower bounds of the 95% CI depicted in Figure 5.6 can be found in Table 5.8. For example, in the 10-15 mph speed range, the lower and upper bounds for PC following trucks class 8 are 3.82 and 4.46 seconds, respectively; for trucks class 8 following PC they are 3.59 and 4.58 seconds, respectively. Since the 95% headway CI for these two Lagging-Leading vehicle pairs overlap, the corresponding means (4.14 and 4.09 seconds) are not statistically significantly different (see overlapping blue (PC) and green (trucks class 8) confidence intervals for the 10-15 mph speed range on Figure 5.6). However, lagging trucks class 8 had statistically significant higher mean headways than lagging PC for all other examined speed ranges. A similar pattern of statistically significant differences was found for lagging trucks class 9, compared to lagging PC, but significant differences started at the 20-25 mph speed range-see Figure 5.7.
TABLE 5.8 Average value and the 95% Confidence Interval of Headway (second) for
different Passenger Cars (PC)- Semi trucks lagging-leading pair. Lagging Vehicle speed range (mph) Lagging-Leading vehicle pair N Mean Std. Deviation Std. Error 95% Confidence Interval for Mean
Lower Bound Upper Bound PC-Truck class 8 93 4.14 1.55 .16 3.82 4.46 10-15 PC-Truck class 9 28 5.51 1.84 .35 4.80 6.22 Truck class 8-PC 53 4.09 1.79 .25 3.59 4.58 Truck class 9-PC 23 5.36 1.97 .41 4.51 6.21 PC-Truck class 8 315 2.97 1.71 .10 2.78 3.16 15-20 PC-Truck class 9 131 4.61 1.25 .11 4.39 4.82 Truck class 8-PC 338 3.50 1.70 .09 3.32 3.68 Truck class 9-PC 130 4.66 1.98 .17 4.31 5.00 PC-Truck class 8 262 2.54 1.45 .09 2.36 2.72 20-25 PC-Truck class 9 161 4.01 1.25 .10 3.81 4.21 Truck class 8-PC 294 3.44 1.89 .11 3.22 3.66 Truck class 9-PC 156 4.77 2.13 .17 4.44 5.11 PC-Truck class 8 222 2.79 1.46 .10 2.60 2.98 25-30 PC-Truck class 9 145 3.60 1.43 .12 3.37 3.84 Truck class 8-PC 210 3.53 1.94 .13 3.27 3.80 Truck class 9-PC 147 4.28 1.93 .16 3.96 4.59 PC-Truck class 8 206 2.52 1.38 .10 2.33 2.71 30-35 PC-Truck class 9 141 3.12 1.12 .09 2.93 3.30 Truck class 8-PC 169 3.57 2.11 .16 3.25 3.89 Truck class 9-PC 129 4.04 1.96 .17 3.70 4.38 PC-Truck class 8 193 2.62 1.47 .11 2.41 2.83 35-40 PC-Truck class 9 143 2.93 1.29 .11 2.71 3.14 Truck class 8-PC 164 3.38 1.98 .15 3.07 3.68 Truck class 9-PC 172 3.54 1.76 .13 3.28 3.81 PC-Truck class 8 322 2.35 1.38 .08 2.20 2.50 40-45 PC-Truck class 9 230 2.77 1.27 .08 2.60 2.93 Truck class 8-PC 274 3.05 1.89 .11 2.82 3.27 Truck class 9-PC 246 3.49 1.91 .12 3.25 3.73 PC-Truck class 8 919 2.49 1.64 .05 2.38 2.59 45-50 PC-Truck class 9 785 2.62 1.55 .06 2.51 2.73 Truck class 8-PC 598 3.03 1.98 .08 2.87 3.19 Truck class 9-PC 1143 3.53 2.11 .06 3.40 3.65 PC-Truck class 8 11279 2.87 2.01 .02 2.83 2.91 >50 PC-Truck class 9 11614 3.11 1.94 .02 3.07 3.14 Truck class 8-PC 11822 3.12 2.15 .02 3.08 3.16 Truck class 9-PC 11230 3.30 2.10 .02 3.26 3.33
48
FIGURE 5.6 95% mean Headway Confidence Intervals for Passenger Cars-following-Trucks class 8 and Trucks class 8-following- Passenger Cars within specified speed ranges.
49
FIGURE 5.7 95% mean Headway Confidence Intervals for Passenger Cars-following-Trucks class 9 and Trucks class 9-following- Passenger Cars within specific speed ranges.
5.5.3 Gross Vehicle Weight-Headway Relation
The preceding section addressed PC-HV pairs in the traffic stream. It documented that lagging larger vehicle drivers (classes 8 and 9) maintain longer headways than lagging PC drivers. The heaviest analyzed vehicles (class 9) maintained the longest headways thus a basic vehicle weight-headway relationship was evident (the heavier the lagging vehicle, the longer the headways). This relationship seems reasonable, since heavier vehicles need longer distances to decelerate, thus their drivers choose to maintain longer headways in order to have adequate deceleration/stopping distances.
The relationship between vehicle class and vehicle weight is not straight- forward, however, due to the effect of payload on the overall vehicle weight (Gross Vehicle Weight-GVW). For PC, the maximum payload amounts to about 20% of the empty vehicle weight. For vehicle classes 3-6, maximum payloads may amount to as much as 50% to 100% of the empty vehicle weight; for semi trucks (classes 8 and 9) maximum payload may be up to 200% of their empty weight. For example, a class 8 vehicle empty weight ranges from 20-26 kips with a maximum payload of 54 kips and a maximum GVW of 80 kips.
Thus, a loaded class 8 vehicle may exceed the typical weight of an empty class 9 vehicle. This example leads to the observation that, if headways are