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Conditions and Performance Report. Appendix B.
Conditions and Performance Report
Appendix B—National Highway System |
| Conditions and Performance
Chapter Listing
Conditions and
Performance Home Page
Introduction
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Chapter 4 introduced "delay" as a measure of highway operational
performance. Delay is a modeled measure calculated as the difference between
estimated average travel speed and free flow travel speed. In this report,
delay is expressed in terms of vehicle-hours of delay per thousand VMT. Overall
delay on the NHS declined from 4.397 to 4.368 hours per thousand VMT
between 1995 and 1997.
Volume/service flow (V/SF) is a measure of the severity of congestion. The
V/SF is the ratio between the volume of traffic actually using a highway during
the peak hour, and the theoretical capacity of the highway to accommodate
traffic. This report has traditionally used a threshold value of 0.80 to
describe the onset of congestion. Between 1995 and 1997, the percent of urban
peak hour travel on the NHS that occurs in congested conditions rose from
44.9 percent to 45.2 percent.
| Q How does delay on the NHS
compare with delay on all arterials and collectors? |
| A Delay per thousand VMT is
lower on the NHS (4.368 hours) than on all arterials and collectors (8.973).
Delay is generally lower on the higher-ordered functional systems that make up
the bulk on NHS mileage. |
| Q How does the percentage
of urban peak-hour congestion on the NHS compare to peak-hour congestion on all
urban principal arterials? |
| A The percent of peak-hour
urban traffic that operates at a V/SF greater than or equal to 0.80 is
higher on the NHS (45.2 percent) than on all urban principal arterials combined
(40.2 percent). The NHS includes the entire Interstate system, and V/SF
ratios on urban Interstates tend to be higher than on other urban principal
arterials. |
| Q How do the conditions and
performance of NHS routes with heavy truck traffic compare to those with fewer
trucks? |
| A Approximately 20 percent of
NHS mileage has truck traffic that exceeds 25 percent of total traffic on these
routes. Exhibit B-8 compares the percent of pavement with acceptable ride
quality and the percent of congested travel for NHS routes with 25 percent or
more trucks with those with lighter truck traffic. As indicated earlier, to
meet the FHWA Strategic Plan standard for acceptable ride quality, pavement
must have an IRI value of 170 or less. In this exhibit, congested travel
includes sections with a V/SF ratio of 0.80 or higher. This exhibit shows that on the NHS pavement is in better condition on routes
with high truck travel than on those with fewer trucks, and the portion of
miles with smooth pavement increased from 1995 to 1997. While heavier vehicles
cause more damage to pavement than lighter vehicles, routes most used by trucks
are typically those with pavement with a higher strength than average, and that
receive more than average attention from the appropriate jurisdictions for
rehabilitation and maintenance.
The exhibit also shows that there is less congestion on routes with a high
percentage of truck travel, but that the congestion on those routes is
increasing. Truck drivers chose routes with less congestion when feasible. |
Exhibit B-8. Conditions
and Performance of NHS Routes with Heavy Truck Traffic
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Page last modified on November 7, 2014
