Assessment of Fuel Economy Technologies for Medium and Heavy ...

Scenarios for estimation of change in delay: A basic four-lane freeway segment (two lanes in either
direction) is at capacity when there are at least 2,200 passenger cars per lane per hour, or 4,400
passenger cars traveling in either direction. The roadway has a free-flow travel speed of 70 mph,
no horizontal obstructions, two 12-foot travel lanes, no grade, and is 10 miles in length. The two
scenarios below illustrate the impact of trucks on highway capacity under a condition of
moderately heavy demand:
Base scenario: A roadway that carries a significant number of trucks and passenger vehicles
but currently operates at an acceptable level of service, and does not experience many hours
of delay. The volume (demand) consists of 3,400 total vehicles on the basic freeway segment
per hour. Ten percent of them (340 vehicles) are trucks and they have, on average, a 500
horsepower engine and a fully-loaded gross-vehicle weight of 80,000 lbs, which produces a
weight to horsepower ratio of 160 lb/hp (80,000/500);
Truck VMT increase scenario: This scenario is identical to the base scenario described above
except that trucks are increased between 2.2 and 10.5 percent to grow with the increase in
traffic described in Section (1)(ii).
Findings:
Base scenario for estimation of change in delay: To estimate the volume to capacity ratio, take the
3,400 vehicles and add the additional PCEs created by the trucks. Each truck on this segment
occupies the same capacity as 2.6 passenger vehicles (From Figure A-1, each truck is worth 2.6
PCEs). The road operates as if there are 3,060 autos plus 884 PCEs (340*2.6), identical to a
scenario where there are no trucks and 3,994 passenger cars. The estimated V/C ratio is
3,944/4,400, or 0.90. Using the BPR formula, the congested speed for this segment is 63.8 mph
((70) / (1 + 0.15 * [3,944/4,400] ^ 4) =63.8 mph). It takes 8.6 minutes to travel the 10-mile
roadway in free-flow conditions and 9.4 minutes in congested conditions, which implies a delay
of 0.8 minutes per vehicle under the operating conditions described in the scenario. For all 3,400
vehicles, this implies a total delay of 2,720 minutes of delay, approximately 45 hours.
Truck VMT increase scenario for estimation of change in delay: In this scenario, truck volumes increase
by 2.2 - 10.5 percent over the base scenario conditions, an increase of 8 - 36 trucks or an increase
in total vehicles of 0.2 – 1.1 percent. The road now operates as though there are 3,060 autos plus
904 - 978 PCEs (348*2.6; 376*2.6), identical to a scenario where there are no trucks and
3,964 - 4,038 passenger cars. The V/C ratio is 3,964/4,400 – 4,038/4,400, or 0.90 - 0.92. Using the
BPR formula, the congested speed for this segment is 63.7 - 63.3 mph. This marginally increases
the travel time for the 10-mile segment to 9.4 - 9.5 minutes in congested conditions, which implies
a delay of 0.8 - 0.9 minutes per vehicle. For all 3,408 - 3,436 vehicles, this implies a total delay of
2,726 - 3,092, approximately 45 - 52 hours – an increase in delay of 0 – 16 percent compared to the
base scenario.
This increase in delay is small compared to the overall travel time on this segment (although it is
valid only at this particular point on the BPR curve.) Furthermore, the majority of VMT on U.S.
freeways and arterial roadways occurs at acceptable levels of service (71 percent of urban VMT
and 92 percent of rural VMT currently operates at level of service D or better) and impacts on
these roadways are likely to be minimal. However, the marginal impact on delay will become
larger as the capacity of the road is approached. For some roadways operating at or near
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