# Presentation: 815225

## Speed on Roadway Segments

- National Forum
- on
*Speeding*
- June 15-16, 2005
- By Kay Fitzpatrick

## Sources of Material for Presentation

- NCHRP Project 15-18
- FHWA Rural Highway Studies
- TxDOT Research
- Literature

## Topic Areas

- Design Speed
- Posted Speed
- Operating
- 	Speed

## Design Speed

## Rural Highway, Horizontal Curves (128 sites, 6 states)

## Suburban Arterial, Horizontal Curves (19 sites, Texas)

## Posted Speed

## Procedures Used to Set Speed Limits (ITE Survey)

- 85th percentile speed – predominant factor
- Roadway geometry
- Roadside development
- Crash experiences
- Political pressure

## Deviations From the 85th Percentile Speed (ITE Survey)

- Politics (33%)
- Crashes (13%)
- Roadway areas (11%)
- Roadway geometry (9%)

## MUTCD Guidelines

- 85th percentile speed of free-flowing traffic rounded up to nearest 5 mph
- Road characteristics
- Speed pace
- Roadside development
- Parking practices
- Reported crash experiences

## 85th Speed vs Speed Limit (NCHRP / TxDOT / FHWA Data)

## Percentile Speed Equals Posted Speed (Rural Roads)

| Source | Functional
    Class | Vehicle Percentile (%) 
      When Speed = |  |  | Sites |
| --- | --- | --- | --- | --- | --- |
|  |  | Posted | + 5 mph | + 10 mph |  |
| NCHRP | Rural, Arterial | 37 / 48* | 70 | 91 | 9 |
| FHWA | Rural, Minor Arterial  | 59 | 87 | 99 | 9 |
| FHWA |  Rural, Principal Arterial | 72 | 90 | 98 | 36 |
| FHWA | Rural  | 64 | 86 | 97 | 126 |

## Percentile Speed Equals Posted Speed (Suburban/Urban Roads)

| Source | Functional
    Class | Vehicle Percentile (%) 
      When Speed = |  |  | Sites |
| --- | --- | --- | --- | --- | --- |
|  |  | Posted | + 5 mph | + 10 mph |  |
| TxDOT | Suburban/Urban   Arterial | 32 | 69 | 91 | 35 |
| NCHRP | Suburban/Urban   Arterial | 32 | 69 | 92 | 35 |
| NCHRP | Suburban/Urban   Collector | 23 | 57 | 86 | 22 |
| NCHRP | Suburban/Urban   Local | 52 | 83 | 96 | 13 |

## Operating and Posted SpeedField Studies

- Previous findings:
- 85th %-ile speed exceeds posted
- 50th %-ile speed near posted
- NCHRP/other studies:
- Rural: 37 to 72% at posted speed
- Suburban/urban: 32 to 52% at posted

## Speed Zoning Reports

- ITE TENC Committee 97-12
- Request “speed zoning investigations your agency has recently conducted”
- 256 report received
- 128 contained both 85th percentile speed value and speed limit recommendation

## Operating and Posted SpeedSpeed Zone Studies

- 128 speed zone studies  10% rounded *up* and 31% rounded to *nearest* 5 mph

-     **Dif (85****th**** – Recom SL)**

## Operating and Posted SpeedComments

- 85th percentile speed is a “starting point”
- Encourage changes in how speed limits are set?

## Operating Speed

## FHWA StudyRural Highways

- FHWA StudyRural Highways

**Notes:**
The general criteria used were selected to result in a database that represents the most common conditions found in the United States.  Several additional criteria were also used in selecting the sites.  When vertical curves were combined with horizontal curves, the points of inflection of both curves could not be more than 30 m apart, and a significant overlap had to exist between the lengths of both curves. Lower volume facilities, less than 2000 vehicles per day, were preferred to reduce the potential for restricted vehicle flow.  Also, the curves selected could not be close to towns or developed areas that may significantly affect the speed patterns on the curves.   

## FHWA Study - Rural HighwaysSpeeds on Horizontal Curves

- FHWA Study - Rural HighwaysSpeeds on Horizontal Curves

**Notes:**
Figure 1 shows that as R increases from 0 to 400 m, the 85th percentile speeds increase notably for all study locations.  For radii greater than 400 m, the increase in speed is not as dramatic. Three of the four equations have intercept values greater than the 97.9 km/h recommended by Krammes et al.(4) on long tangents.  Therefore, in certain situations, the equations would predict speeds higher than the assumed speed on a long tangent.  Observed speeds on long tangents from this study ranged from 93 to 104 km/h (average 85th percentile speed, by state).  Based on the data and engineering judgement, the maximum operating speed on horizontal curves and tangents could be rounded to 100 km/h.  Thus, operating speeds on large radius horizontal curves should be truncated to 100 km/h (or to another desired operating speed) when the predicted speed exceeds this value. 

## FHWA Study - Rural HighwaysLimited Sight Distance Curves

- FHWA Study - Rural HighwaysLimited Sight Distance Curves

**Notes:**
Ten study sites were available from the current study and 32 sites from the SSD study for the evaluation of the LSD sites.  The  review of the correlation matrix found 1/K to be correlated with the 85th percentile speeds.  Regression analysis showed that 1/K was significant (0.033), however the R2-values was extremely low (0.10).  A possible reason for the low R2 was that there was a study location where the posted speed was 112.7 km/h, resulting in a large variation in 85th percentile speed.  While all the SSD sites had a speed limit of  88.5 km/h, the data were collected prior to the repeal of the nationally mandated speed limit and there was an observable difference in the plot of the data.  The SSD speed data were consistently lower then the speed data observed in the current research.  Therefore, the SSD data were not included in the final equation development.  The equation was developed from the nine observations where the posted speed was less than or equal to 88.5 km/h. Both the intercept (0.0001) and the 1/K variable (0.0134) were significant. 

## FHWA Study - Rural HighwaysFindings

- Horizontal Alignment
- Radius: key variable
- Other variables: length of curve, deflection angle, superelevation, desired speed (previous tangent speed)
- Vertical Alignment
- K, SSD
- Combination Alignments
- Theories

**Notes:**
Horizontal Alinement.  Speed prediction on rural two-lane roads has been researched extensively for horizontal curves on relatively flat terrain. Previous research indicates that curve radius is the most important element in determining speeds on horizontal curves.  Generally, speed is reduced as the radius is reduced. Pavement width, lane width, and shoulder width may have an effect on operating speeds.  
Vertical Alinement. Vertical curves in the United States are parabolic and are described by the rate of vertical curvature, K, which is the horizontal distance in meters required to effect a one percent change in gradient.  For crest vertical curves, stopping sight distance is a key variable. Fambro et al’s study of 42 curves in three states also concluded that the inferred design speed of a crest curve (with narrow shoulders) is a moderately good predictor of 85th percentile speeds for these types of roadways.  The design speed was inferred using the rate of vertical curvature (K) and a two-lane rural road was classified as having narrow shoulders if the shoulder width was less than 1.8 m.  Grades and length of the grade have a physical effect on speeds and should be considered in a model used to estimate operating speeds. 
Combination of Horizontal and Vertical Alinement.  The combination of horizontal and vertical alinement on speed has not been systematically studied.  Most studies have been directed towards either horizontal or vertical alinements individually.  In theory, the combination of horizontal and vertical curves could affect operating speed in the following ways: (1) Horizontal and vertical alinement act independently of each other and their effect is linear and additive. (2) The effect of horizontal or vertical alinement depends on the value of the other and there is an interact between them.  (3) The most severe alinement condition controls and the other has no marginal effect and can be ignored.

## NCHRP Field Studies

- 79 tangent sites, most suburban/urban
- Sites not near signals or horizontal curves (elements known to influence operating speed)
- Free-flow speed measured with laser and counters
- Roadway and roadside characteristics

## NCHRP StudyPosted Speed Limit

- NCHRP StudyPosted Speed Limit

## NCHRP StudyRoadway Type

**Cum Dist (%)**

- NCHRP StudyRoadway Type

## NCHRP StudyAccess Density

- 15

- 30

- 45

- 60

- 75

- 0

- 30

- 60

- 90

- 120

- 150

- 180

**Access Density (pts/mi)**

** ****85****th**** %-ile Speed (mph) **

- Local

- Collectors

- Arterial, C&G

- Arterial, Shoulders

- NCHRP StudyAccess Density

## NCHRP StudyParking

## NCHRP StudyPedestrian Activity

## NCHRP StudyCenterline Markings

## NCHRP StudyMedian Width

## NCHRP StudyTotal Pavement Width

## 
Several variables show influences:
Access density
Pedestrian activity
Absence of centerline or edge line markings
On-street parking
Median presence
Additional data needed

- Several variables show influences:
- Access density
- Pedestrian activity
- Absence of centerline or edge line markings
- On-street parking
- Median presence
- Additional data needed

- NCHRP StudyInfluence on Operating Speed

## Summary

## Influence on Horizontal Curve Speed

|  | Pre 1990s
Rural | Krammes
1993
Rural | Fitz
1999
Rural | Schurr
2002
Rural | Poe
1996
Low | Fitz
2000
Urban |
| --- | --- | --- | --- | --- | --- | --- |
| DC / Radius | Y | Y | Y |  | Y |  |
| Deflect |  | Y |  | Y |  | Y |
| Len Curve |  | Y |  | Y |  |  |
| Inferred Sp |  |  |  |  | Y |  |
| Lane Width |  |  |  |  | Y |  |
| Access |  |  |  |  | Y | Y |
| Speed Limit |  |  |  |  | Y | Y |
| Grade |  |  | Y | Y | Y |  |

## Influence on Tangent Speed

|  | Parma
Rural
1999 | Dixon
Rural
1999 | Polus
Rural
2000 | Fitz TX Urban
2000 | Fitz TX
Urban
2000 | Fitz
Urban
2003 |
| --- | --- | --- | --- | --- | --- | --- |
| Lane Width |  |  |  |  | Y |  |
| Nearby HC |  |  | Y |  |  | O |
| Access | SL | O | SL |  |  | Y |
| Speed Limit |  |  |  | Y | Not | Y |
| Grade | Y | O |  | SL | SL |  |
| Pedestrian |  |  |  |  |  | O |
| Median |  |  |  |  |  | O |
| Parking |  |  |  |  |  | O |
| SL=study limited, O = may, Y = yes |  |  |  |  |  |  |

## Questions?