U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
| REPORT |
| This report is an archived publication and may contain dated technical, contact, and link information |
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| Publication Number: FHWA-HRT-17-098 Date: January 2018 |
Publication Number: FHWA-HRT-17-098 Date: January 2018 |
The following case studies seek to illustrate the relationship between the inferred design speed, designated design speed, and operating speed on two-lane rural highways and describe the use of the various self-enforcing roadways design methods. These methods include the inferred design speed approach, the IHSDM DCM, and USLIMITS2. The other two methods described in the previous section—applying geometric design criteria and employing a combination of signs and pavement markings—will not be applied to the case study examples. The establishment of limiting values for all speed-based geometric design criteria has not been thoroughly evaluated due to limited empirical research on this topic. As such, this method is not applied to the case study examples. The combination of signs and pavement markings method must be applied to roadways, and operating speeds before and after implementation should be collected and analyzed to assess the effectiveness of the markings.
Each of the methods in this section of the report is demonstrated using two case study examples. The first example, US Route 6 in Pennsylvania, illustrates speed harmony on a self-enforcing roadway. The second example, SR 865 in Virginia, demonstrates speed discord and, therefore, is not considered a self-enforcing roadway. The case studies also include information about the roadway characteristics (e.g., horizontal and vertical alignment data, cross-section information, and traffic control devices) and measured operating speeds collected at the sites. The following sections illustrate the self-enforcing road-design approaches for the two case studies.
The US Route 6 study segment is a two-lane, rural principal arterial in Sheffield, Pennsylvania, approximately 1.4 mi (2.3 km) in length. Figure 59 is a map of the study segment, and figure 60 is a plan view of the study segment with the speed data collection locations noted. Data were collected in the eastbound direction of travel using on-pavement sensors. The segment includes seven simple horizontal curves with radii ranging from 716 to 1,432 ft (218.2 to 436.5 m) and five access points, three of which are low-volume, unpaved driveways; the other two access points serve a natural gas plant located on both sides of the highway at the east end of the segment. There are also five crest vertical curves. When the roadway was built in 1925, superelevation was provided on horizontal curves, although no design speed was designated. A subsequent project (completed in 2004) employed a designated design speed of 60 mph (96.6 km/h) and increased the superelevation at each horizontal curve. The vertical alignment has not been altered since construction in 1925. The available sight distance for one crest vertical curve located near the middle of the study segment is less than the criteria associated with a 60-mph (96.6 km/h) design speed. The maximum grade within the segment is 7 percent. The posted speed limit is 55 mph (88.5 km/h), but there are no speed limit signs within the study segment.
As shown in figure 61, the typical cross section includes one travel lane in each direction and a paved shoulder. The clear zone is narrow with a guardrail adjacent to the shoulder along much of the eastbound side. The adjacent land is wooded with a natural gas plant located at the eastern end of the study segment. There are no designated pedestrian facilities in the study segment. Signs indicate that the roadway is a bicycle corridor. The AADT for the study segment is approximately 2,500 vehicles/d, of which 10 percent is heavy vehicles. There were no observed pedestrians or bicycle traffic during the data collection period.
©2016 Google®; annotations by The Pennsylvania State University and Institute of Transportation Engineers.
Figure 59. Map. US Route 6 study segment map.
The inferred design speed approach was used to create an inferred design speed plot and compare the designated design speed, posted speed limit, and 85th-percentile operating speeds based on the field data collected. Figure 62 is the generated speed profile for the study segment (direction of travel is left to right). At numerous locations, sight distances are limited by the vertical and horizontal alignment combined with lateral obstructions, mostly cut slopes. Sight distance restrictions translate to inferred design speeds as low as 39 mph (62.8 km/h). The inferred design speeds for horizontal curves range from 48 to 61 mph (77.2 to 98.2 km/h). A maximum inferred design speed was used at some locations, typically along tangents. On these sections, there are no geometric features to limit operating speeds, which could be high. The vertical axis of the speed profile plot shown in figure 62 was truncated at 80 mph (128.7 km/h) to show the variability in the operating speeds along the entire study segment and to fit the speed profile on a single page. An advisory speed of 40 mph (64.4 km/h) is posted at four horizontal curves and an advisory speed of 50 mph (80.5 km/h) is posted at two other curves. The advisory speeds are shown on the speed profile plot along with the 85th-percentile operating speeds for each successive geometric element (horizontal curve and tangent).
As shown in figure 62, speed harmony generally exists between the posted speed limit, designated design speed, and 85th-percentile operating speeds. Speed harmony is an example of a self-enforcing roadway. The 85th-percentile operating speed exceeds the designated design speed at only one location, a horizontal curve. The measured speeds on this horizontal curve are higher than on the approach tangent. The inferred design speed is well below the designated design speed at two locations due to SSD restrictions. Sight distance is limited at station 40+00 by a crest vertical curve and by lateral obstructions between stations 60+00 and 70+00, which results in the 85th-percentile speeds exceeding the inferred design speed by 15 to 20 mph (24.1 to 32.2 km/h).
The IHSDM DCM was used to identify any possible speed inconsistencies for the US Route 6 case study. The operating speed model can help determine any potential speed management or safety issues along the roadway. Data input to the IHSDM include speed information (e.g., desired speed, design speed, and posted speed limit for lower speed highways), horizontal curvature information, vertical curvature information, and information on the surrounding area type (e.g., rural, suburban, or urban). Figure 63 shows the IHSDM DCM output. The US Route 6 geometric elements, desired speed, design speed, and 85th-percentile operating speeds are included in the plot.
In figure 63, the green flags show that the speed differential between adjacent design elements is less than 6 mph (9.7 km/h). This indicates consistency among adjacent design elements; no sharp speed reductions between elements are required. Table 17, table 18, and table 19 display the results from the IHSDM DCM in tabular format.
Condition 1 = 0 mph (0 km/h) ≤ (V85 - Vdesign) ≤ 6 mph (9.7 km/h);
Condition 4 = (V85 - Vdesign) < 0 mph (0 km/h); V85 = estimated 85th-percentile operating speed;
Vdesign = design speed.
Condition 1 = 0 mph (V85,Tangent - V85,Curve) ≤ 6 mph (9.7 km/h); V85,Tangent = estimated 85th-percentile operating speed on tangent;
V85,Curve = estimated 85th-percentile operating speed at the beginning of the curve.
As shown, the speed differential between the estimated 85th-percentile operating speed and the design speed (table 18) and the speed differential between the estimated 85th-percentile operating speed on tangent and the estimated 85th-percentile operating speed at the beginning of the curve (table 19) are all less than 6 mph (9.7 km/h). Similar to figure 63, table 17, table 18, and table 19 also indicate that the posted speed limit, 85th-percentile operating speeds, and design speed are in harmony. Therefore, US Route 6 is considered a self-enforcing, or self-explaining, roadway.
The USLIMITS2 Web-based software was used to determine a recommended speed limit for US Route 6 based on various features of the roadway and the surrounding area. Table 20, table 21, and table 22 present the information inputted into USLIMITS2 for US Route 6.
*A roadside hazard rating of 3 and 5 both produce the same recommended speed limit.
| Traffic Factor | Traffic Information |
|---|---|
| 85th-percentile speed | 57 mph (91.7 km/h) |
| 50th-percentile speed | 52 mph (83.7 km/h) |
| AADT | 2,500 vehicles/d |
Using the inputs in table 20, table 21, and table 22, USLIMITS2 produced a recommended speed limit of 55 mph (88.5 km/h) for the US Route 6 study segment. This is consistent with the actual posted speed limit of US Route 6, which is also 55 mph (88.5 km/h).
The design of US Route 6 produces operating speeds that are consistent with the posted speed limit and the designated design speed. The geometric design, which contributes to the presence of speed harmony for this roadway segment, is a variation of horizontal and vertical curves, including sight distance restrictions. These geometric features reduce the drivers ability to operate motor vehicles at speeds that exceed the posted or designated design speed. Another respect in which US 6 operating speeds are consistent with the posted speed limit and designated design speed concerns the correlation between the designated design speed and the posted speed limit. The designated design speed and the posted speed limit differ by 5 mph (8.0 km/h). The designated design speed is 60 mph (96.6 km/h), and the posted speed limit is 55 mph (88.5 km/h). Operating speeds tend to fall within this 5-mph (8.0 km/h) gap. The speed limit appears to be set in accordance with the 85th-percentile operating speeds and is a typical limit for the area type surrounding the roads and for a two-lane rural highway.
Ten yr (2005 through 2014) of crash data for US Route 6 were compiled to assess the historical safety performance of this roadway. The study reported on crashes from the period, with three of them being fatal. Table 23 shows the breakdown per year for the reported crashes occurring on US Route 6.
The IHSDM contains a module that predicts crashes based on the geometric elements of the roadway, the surrounding area, and traffic conditions, such as AADT. This crash prediction module was used to predict crashes for the years 2005 through 2014. Crashes were predicted sing the module with and without the empirical Bayes (EB) method. The crash predictions using the EB method were completed using the HSM calibration factor and then using a county-specific calibration factor for District 1 in Pennsylvania, where this site is located. The calibration factor for District 1 in Pennsylvania is 1.05. These predicted numbers were compared to the reported crash data for the same 10-yr period. Table 24 compares the historical reported crash data with the predicted past crashes.
As shown in table 24, the predicted number of crashes from the IHSDM is approximately double the number of reported crashes identified through historical crash data. The actual historical crash data indicate that a total of eight crashes (with three being fatal-plus-injury crashes) occurred in 10 yr. This translates to a crash rate (for total crashes) of 0.571 crashes per mile (1.6 km), per year. When the EB method was applied using the calibration factor for District 1 in Pennsylvania, the predicted number of crashes is closer to the reported number of crashes. This analysis and comparison between historical crashes and predicted crashes from the IHSDM shows that speed management is important. In this particular case study, a roadway that exhibits speed harmony has fewer reportable crashes than predicted by the IHSDM crash prediction module. Because this is only a single case study, additional studies should be undertaken to determine if this relationship exists on rural two-lane highways.
The study segment is a nearly 1-mi (1.6-km) section of a two-lane rural major collector. Figure 64 is a map of the study segment location, and figure 65 is a plan view of the study segment with the speed sensor locations noted. Data were collected in the northbound direction of travel. The study segment has five horizontal curves with radii ranging from 477 to 3,418 ft (145.4 to 1,041.8 m). The second and third horizontal curves constitute a reverse curve, and the third and fourth curves are compound curves. The result is a series of three contiguous curves, unseparated by tangents. The designated design speed is 40 mph (64.4 km/h). The posted speed limit is 45 mph (72.4 km/h) in the study segment, with one advisory speed of 35 mph (56.3 km/h) in place for a school zone. The first and last curves on the study segment narrowly exceed the minimum criteria for the designated design speed of 40 mph (64.4 km/h). The three consecutive curves have larger radii, and the inferred design speeds well exceed the designated design speed. State Routes 828 and 865 form a four-way intersection, one leg being the entrance to an elementary school. Left-turn lanes are provided on both Rockfish Road approaches. The average segment access density is 30.6 access points per mile consisting primarily of residential driveways.
The typical cross section of the segment, shown in figure 66, consists of one travel lane in each direction flanked by unpaved earthen shoulders. There are no facilities for pedestrian or bicycle traffic. Detached, single-family homes are moderately spaced along both sides of the alignment and set back over 50 ft (15.2 m) from the roadway. An elementary school is located in the approximate middle of the study segment. The clear zone is about 30 ft (9.1 m) for most of the alignment. The guardrail is located along the first and third horizontal curves and separated from the edge of the traveled way by approximately 10 ft (3.0). The average daily traffic on the study segment is approximately 600 vehicles/d, of which 4 percent is heavy vehicles. There was no observed pedestrian or bicycle traffic during the data collection period. Additionally, operating speeds were collected at consecutive horizontal curves and tangents throughout the study segment.
©2016 Google®; annotations by The Pennsylvania State University and Institute of Transportation Engineers.
Figure 64. Map. SR 865 (Rockfish Road) study segment map.
The inferred design speed approach was used to create an inferred design seed plot and compare the designated design speed, posted speed limit, and operating speeds based on field data. Figure 67 shows a speed profile plot for Rockfish Road (direction of travel is left to right).
As shown in figure 67, speed discord generally exists between the posted speed limit, designated design speed, and 85th-percentile operating speeds. The geometric design of the SR 865 study segment produces operating speeds that are higher than the posted speed limit. This is not an example of a self-enforcing, or self-explaining, roadway. Inferred design speeds range from 41 to 92 mph (66.0 to 148.1 km/h). The posted speed limit exceeds the designated design speed, and the posted speed limit also exceeds the inferred design speed along two horizontal curves. These two horizontal curves appear to be associated with lower operating speeds when approaching the horizontal curves. The 85th-percentile operating speeds are close to the posted speed limit near the beginning and end of the study segment where the inferred design speeds are near the designated and posted speeds. Observed speeds increase beyond the school zone, which has a posted advisory speed limit of 35 mph (56.3 km/h).
The IHSDM DCM was used to identify any possible speed inconsistencies for SR 865. The operating speed model can help determine any potential design consistency issues along the roadway. The inputs into the IHSDM include speed information (e.g., desired speed and design speed, and the posted speed limit for lower speed highways), horizontal curvature information, vertical curvature information, and information on the surrounding area type (e.g., rural, suburban, or urban). Figure 68 shows the IHSDM DCM output. The SR 865 geometric elements, desired speed, designated design speed, and 85th-percentile operating speeds are shown on the plot.
Source: FHWA.
Note: 1 mph = 1.60934 km/h.
Figure 68. Illustration. SR 865 (Rockfish Road) IHSDM output.
In figure 68, the green flag shows that the operating speed differential between adjacent design elements is less than 6 mph (9.7 km/h). The yellow flag indicates that the speed differential between adjacent design elements is greater than or equal to 6 mph (9.7 km/h and less than or equal to 12 mph (19.3 km/h). The red flag shows that the speed differential between adjacent design elements is greater than 12 mph (19.3 km/h). The presence of yellow and red flags indicates there is no consistency among the geometric elements on the SR 865 case study site.
While the flags in figure 68 represent the design element differential, the color-coded speed profile represents the 85th-percentile operating speed differential, which is the speed differential between the design speed and the 85th-percentile operating speed. The orange portions of the profile line represent speed differentials that are less than or equal to 12 mph (19.3 km/h), and the red portions of the speed profile lines represent speed differentials that are greater than 12 mph (19.3 km/h). Both the red and orange portions of the line indicate that the difference between the design speed and the 85th-percentile operating speed is large, which suggests there is no consistency among the design elements. Table 25, table 26, and table 27 display the results from the IHSDM DCM in tabular format.
*The deceleration rate predicted for the range(s): [0.000 to 2+77.040] (in the direction of increasing stations) is
greater than the approximated comfortable deceleration rate as determined by data collected to develop the DCM
(as referenced in FHWA Report No. FHWA-RD-99-171, Speed Prediction for Two-Lane Rural Highways).
Condition 2 = 6 mph (9.7 km/h) < (V85 - Vdesign) ≤ 12 mph (19.3 km/h);
Condition 3 = 12 mph (19.3 km/h) < (V85 - Vdesign);
V85-estimated 85th-percentile operating speed;
Vdesign = design speed.
Condition 1 = 0 mph (0 km/h) (V85,Tangent - V85,Curve) ≤ 6 mph (9.7 km/h);
Condition 2 = 6 mph (9.7 km/h) < (V85,Tangent - V85,Curve) ≤ 12 mph (19.3);
Condition 3 = 12 mph (19.3 km/h) < (V85,Tangent - V85,Curve);
V85,Tangent = estimated 85th-percentile operating speed on tangent;
V85,Curve = estimated 85th-percentile operating speed at the beginning of the curve.
As shown, the speed differential between the estimated 85th-percentile operating speed and the designated design speed (table 26), and the speed differential between the estimated 85th-percentile operating speed on tangent and the estimated 85th-percentile operating speed at the beginning of the curve (table 27) are all greater than 6 mph (9.7 km/h). Similar to figure 68, table 25, table 26, and table 27 also indicate there are inconsistencies between the posted speed limit, 85th-percentile operating speeds, and the designated design speed. Speed discord occurs; therefore, SR 865 is not considered a self-enforcing, or self-explaining, roadway.
The USLIMITS2 Web-based software was utilized to determine a recommended posted speed limit for SR 865 based on the existing roadway features and the surrounding area.
Table 28, table 29, and table 30 show the information that was inputted into USLIMITS2 for the SR 865 study site.
*A roadside hazard rating of 2 and 3 both produce the same recommended speed limit.
Using the inputs from table 23, USLIMITS2 produced a recommended posted speed limit of 50 mph (80.5 km/h) for the SR 865 (Rockfish Road) study segment. The posted speed limit on SR 865 is 45 mph (72.4 km/h), which is less than the value recommended by the USLIMITS2 expert system.
Five yr (2011 through 2015) of crash data for SR 865 (Rockfish Road) were compiled to assess the historical safety performance of this roadway. On the study segment, there were a total of four crashes from the period, none of which was fatal. Table 31 shows the annual crash frequency distribution for the SR 865 case study segment.
| Year | Total Crashes |
Fatal and Injury Crashes |
|---|---|---|
| 2011 | 0 | 0 |
| 2012 | 1 | 0 |
| 2013 | 2 | 1 |
| 2014 | 1 | 1 |
| 2015 | 0 | 0 |
| Total | 4 | 2 |
The crash prediction module of the IHSDM was utilized to predict crashes for the years 2011 through 2015. Crashes were predicted using the IHSDM crash prediction module with the EB method and the HSM calibration factor. Those predicted numbers were compared to the reported crash data for the same 5-yr period. Table 32 compares the reported crashes to the predicted crashes.
| Years | Total Crashes |
Fatal and Injury Crashes |
|---|---|---|
| Reported crashes during 2011-2015 | 4 | 2 |
| IHSDM crash prediction using EB method | 3.43 | 1.36 |
As shown in table 25, the predicted number of crashes from the IHSDM is approximately the same as the number of reported crashes during the 5-yr analysis period. However, the number of predicted crashes is slightly less than the number of reported crashes. A total of four crashes were reported from 2011 through 2015, while the IHSDM with the EB adjustment predicted 3.43 crashes. This indicates that more crashes were reported than were expected on this case study segment.
Candidate methods to produce consistency between operating speeds and the posted speed limit include geometric changes to the roadway (if reconstruction is planned), or application of signs and pavement markings that can be applied to the existing roadway. Some of the signs and pavement markings currently on the SR 865 study segment include a posted speed limit sign, school warning pavement markings, and centerline and shoulder pavement markings. Other signs and pavement markings, in addition to the current signs and pavement markings, could be introduced in the SR 865 study segment that might possibly reduce speeds and produce operating speeds that are consistent with the posted speed limit.
Based on high-speed traffic calming treatments, various signs and pavement markings that could reduce speeds on tangent sections of the study segment include transverse pavement markings with speed feedback signs. In a study, this combination of sign and pavement markings has shown to have the potential to reduce 85th-percentile speeds by 4 mph (6.4 km/h). (Boodlal et al. 2015) Adding these treatments to the tangent sections could reduce speeds on the tangents and reduce speeds of motor vehicles traveling into the horizontal curves. The transverse pavement markings could create an illusion that drivers are operating at speeds faster than they actually are. This could then possibly reduce driving speeds. The speed feedback sign would inform drivers of their speed and especially draw attention to drivers who are traveling over the posted speed limit. If a speed feedback sign is added solely, without pavement markings, it has displayed the ability to reduce 85th-percentile speeds from 2 to 7 mph (3.2 to 11.3 km/h). (Boodlal et al. 2015)
In addition to the treatments mentioned previously that aim to reduce speeds on tangent sections of SR 865, other treatments could be added to reduce operating speeds on horizontal curves. Such treatments include curve warning signs in combination with chevron signs that span the horizontal curve. While no speed reduction effectiveness evaluation was performed, they pose the potential to reduce operating speeds on horizontal curves. (Boodlal et al. 2015) Adding these signs would alert drivers to the approaching horizontal curve so they could reduce their speeds. Additionally, when an advisory speed limit sign was used for certain geometric conditions that constitute a lower speed, such as a sharp curve, a 15-percent reduction in 85th-percentile speeds was observed. (Boodlal et al. 2015)
The signs and pavement markings mentioned in this section have the potential to reduce speeds on the SR 865 study segment. The treatments vary between being suitable for application on tangent sections or on curve sections of the roadway. Each sign or pavement marking can be used on its own, or signs and pavement makings can be used in combinations to possibly produce operating speeds that are consistent with the posted speed limit.
