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                        Methods and Practices for Setting Speed Limits: An Informational Report


            
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                Appendix D: Northwestern Speed Zoning Technique
The Northwestern Speed Zoning Technique is an example of an engineering method that can be used to calculate a recommended speed limit for a particular facility. It is based on the 85th percentile operating speed, and uses adjustments for different traffic, roadway, and performance characteristics.

The general sentiment in the Northwestern Speed Zoning Technique is that the 85th percentile speed is a safe, self-selected speed that also provides a reasonable basis for enforcement. However, it must be recognized that the driver selects a speed based on her/his evaluation of the perceived hazard, and if there are hazards of which the driver is unaware, then the selected speed may be too high. The commonly encountered hazards and the crash history of the road can be used to determine if the 85th percentile speed is a suitable legislated speed limit.

The Northwestern Speed Zoning Technique provides adjustments for various features of the road—these are generally applicable to most roads, but can be altered to suit local conditions and policies.

The procedure consists of two parts—a minimum study and a detailed analysis. The minimum study is always carried out; the detailed analysis is undertaken when unusual road or land use characteristics make the speed limit as determined by the minimum study seem inappropriate.

**Minimum Speed Study**

The data required for the minimum speed study are:

Speed data:
	85th percentile speeds
		Upper limit of the 15 km/h pace
		Average test run speed
	Physical Road data: » Design speed
	Length of the proposed speed zone
		Average distance between intersections (not including alleys, driveways, or entrances unless they are controlled by STOP signs or traffic signals).The procedure used in the minimum speed study is to determine the speed limit based on the speed data, subject to a maximum as determined by the physical features of the road.

The steps are as follows:

1. For each of the three speed measurements, use Table 18 to select the justified speed limit.

2. Compute a weighted average speed limit using the following weights, and round down to the nearest 10 km/h:

Justified speed limit from the 85th percentile speed: Weight = 3.
	Justified speed limit from the upper limit of the pace: Weight = 3.
	Justified speed limit from the average test run speed: Weight = 4.Please see Extended Text Description below.

(*Extended Text Description:* Equation: *SL* = (3*SL85*+*3SLpace*+*4SLrun*) / 10)



Where: SL = Weighted average speed limit

*SL85*= Speed limit justified by the 85th percentile speed using Table 18

*SLpace*= Speed limit justified by the upper limit of the 15 km/h pace using Table 18

*SLrun*= Speed limit justified by the average test run speed using Table 18

3. Using Table 19, select the highest speed limit that will satisfy all three conditions of design speed, average distance between intersections, and length of the proposed speed zone.

4. The recommended speed limit is the lower of the weighted average (from Step 2) and the maximum speed limit (from Step 3).



**Table 18. Speed Limit Justified by Speed Data**

**85th Percentile Speed (km/h)**
			**Upper Limit of the 15 km/h Pace**
			**Average Test Run Speed (km/h)**
			**Justified Speed Limit (km/h)**< 34
			< 33
			< 30
			3034 - 44
			33 - 42
			30 - 38
			4045 - 54
			43 - 52
			39 - 48
			5055 - 64
			53 - 62
			49 - 56
			6065 - 74
			63 - 72
			57 - 65
			7075 - 84
			73 - 80
			66 - 75
			8085 - 94
			81 - 88
			76 - 85
			9095 - 104
			89 - 96
			86 - 94
			100> 104
			> 96
			> 94
			110

**Table 19. Speed Limit Based on Road Parameters**

**Design Speed (km/h)**
			**Average Distance Between Intersections (m)**
			**Length of Proposed Zone (km)**
			**Maximum Speed Limit (km/h)**110
			400
			1.5
			110100
			300
			1.0
			10090
			250
			0.8
			9090
			175
			0.7
			8070
			125
			0.6
			7070
			100
			0.5
			6050
			75
			0.4
			5050
			60
			0.3
			4030
			45
			0.2
			30**Detailed Analysis**

The data required for the minimum speed study are:

1. Using the recommended speed limit from the Minimum Speed Study and the other collected data, consult Tables 3 to 11 and determine the adjustment factors based on additional traffic and roadway features.

2. Add all of the adjustment factors together to obtain an overall adjustment factor.

3. Calculate the multiplier as follows:

Please see Extended Text Description below.

(*Extended Text Description:* Equation: *MF* = (100+*OAF*) / 100)

Where: *MF*= Multiplication Factor

*OAF*= Overall Adjustment Factor (from Step 2)

4. If the Multiplication Factor is greater than 1.25, set it to 1.25. If the Multiplication Factor is less than 0.75, set it to 0.75.

5. Multiply the recommended speed limit from the minimum speed study by the multiplication factor and round to the nearest 10 km/h to produce the recommended speed limit.

Note: Table 20 will yield two adjustment factors—one for commercial, and one for non-commercial driveways.

Also, if a detailed study is to be undertaken, then all of the information and tables must be included in the analysis. It is not good practice to include selected items and ignore others.



**Table 20. Adjustment Factors for Access Density**

**No. of Driveways per kilometer**
			**Speed Limit from Minimum Study (km/h)****Non-Commercial**
			**Commercial**
			**30**
			**40**
			**50**
			**60**
			**70**
			**80**
			**90**
			**100**
			**110**0 - 3
			0
			+ 15
			+ 15
			+ 15
			+ 10
			+ 10
			+5
			+5
			0
			04 - 6
			0
			+ 10
			+ 10
			+ 10
			+5
			+5
			0
			0
			0
			-57 - 12
			1
			+ 10
			+ 10
			+5
			+5
			0
			0
			0
			-5
			-513 - 21
			2 - 3
			+5
			+5
			0
			0
			0
			-5
			-5
			-10
			-1022 - 30
			4 - 5
			+5
			0
			0
			0
			-5
			-10
			-10
			-15
			-15> 30
			> 5
			0
			0
			-5
			-10
			-10
			-15
			-15
			-20
			-20

**Table 21. Adjustment Factors for Lane Width**

**Lane width (m)**
			**Speed Limit from Minimum Study (km/h)****30**
			**40**
			**50**
			**60**
			**70**
			**80**
			**90**
			**100**
			**110**< 2.8
			0
			0
			0
			-5
			-5
			-10
			-10
			-10
			-152.8 - 3.2
			+5
			+5
			0
			0
			0
			-5
			-5
			-5
			-103.3 - 3.5
			+ 10
			+ 10
			+5
			+5
			0
			0
			0
			0
			-5> 3.5
			+ 15
			+ 15
			+ 10
			+ 10
			+5
			+5
			+5
			0
			0

**Table 22. Adjustment Factors for Functional Classification**

**Functional Classification (Urban Areas Only)**
			**Speed Limit from Minimum Study (km/h)****30**
			**40**
			**50**
			**60**
			**70**
			**80**
			**90**
			**100**
			**110**Local
			0
			0
			0
			-5
			-10
			-10
			-15
			-15
			-20Collector
			+5
			0
			0
			0
			-5
			-5
			-10
			-10
			-15Arterial
			+ 10
			+5
			+5
			0
			0
			0
			-5
			-5
			-10Expressway
			+ 15
			+ 10
			+ 10
			+5
			0
			0
			0
			0
			-5Freeway
			+25
			+20
			+ 15
			+ 10
			+5
			+5
			0
			0
			0

**Table 23. Adjustment Factors for Median Type**

**Functional Classification**
			**Median***None*
			*Flush or Painted*
			*Mountable*
			*Barrier*
			*Depressed Unpaved***0.6m -1.8m**
			**> 1.8m**
			**0.6m -1.8m**
			**> 1.8m**
			**0.6m -1.8m**
			**> 1.8m**
			**1.8m -6.0m**
			**> 6.0m**Local
			0
			+5
			+ 10
			—
			—
			—
			—
			—
			—Collector
			0
			+5
			+5
			+ 10
			+ 15
			—
			—
			—
			—Arterial
			-10
			0
			0
			+5
			+ 10
			+ 15
			+20
			—
			—Expressway
			—
			-10
			-5
			0
			0
			+5
			+ 10
			+ 15
			+20Freeway
			—
			—
			-10
			-10
			-5
			0
			0
			0
			0

**Table 24. Adjustment Factors for Shoulder Type and Width**

**Shoulder Type****Functional Classification**
			**None**
			**Turf or Gravel**
			**Stabilized**
			**Paved**Local
			0
			+5
			+ 10
			+20Collector
			0
			0
			+5
			+ 10Arterial
			-5
			0
			0
			+5Expressway
			-10
			-5
			0
			0Freeway
			-20
			-10
			-5
			0

**Table 25. Adjustment Factors for Pedestrian Activity**

**Pedestrian Activity**
			**Sidewalk Setback from Edge of Pavement (m)****None**
			**0 - 0.5**
			**0.6 - 2.5**
			**2.6 - 4.5**
			**> 4.5****Age < 12**Heavy
			-25
			-20
			-15
			-10
			-5Medium
			-20
			-15
			-10
			-5
			0Light
			-15
			-10
			-5
			0
			0If none, consider ages over 12

			**Age > 12**Heavy
			-10
			-5
			0
			0
			0Medium
			-5
			0
			0
			0
			0Light
			-5
			0
			0
			0
			0None
			0
			0
			0
			0
			0

**Table 26. Adjustment Factors for Parking Activity**

**Functional Classification**
			**Parking Activity****No Parking**
			**Low Turnover**
			**Medium Turnover**
			**High Turnover**Local
			+ 10
			0
			-10
			-10Collector
			+ 10
			0
			-10
			-15Arterial
			+ 15
			0
			-10
			-15Expressway
			0
			-10
			-15
			-20

**Table 27. Adjustment Factors for Roadway Alignment**

**Number of Curves per Kilometer with Advisory Speed < Speed Limit from Minimum Study**
			**Vertical Alignment****Level**
			**Rolling**
			**Hilly**
			**Mountainous**0
			+ 10
			+5
			0
			01
			0
			0
			-5
			-52
			-10
			-10
			-10
			-10> 2
			-20
			-20
			-20
			-20

**Table 28. Adjustment Factors for Crash Rate**

**Crash Rate as a Percent of Area-wide Rate for Similar Facilities**
			**Adjustment**< 75%
			+ 1076% - 125%
			0126% - 200%
			-10> 200%
			-20

**Example Calculation**

The following is an example calculation using the Northwestern Speed Zoning Technique:

Input Data: 85th Percentile Speed = 66.4 km/h

The 15 km/h pace = 45 to 60 km/h

Average test run speed = 56 km/h

Design Speed = 100 km/h

Average Intersection Spacing: 200 meters

Length of Proposed Speed Zone = 0.6 kms



For the minimum speed study, the speed measurements yield the following:

**Criteria**
			**Justified Speed Limit (from Table 18)**
			**Weight**
			**Weighted Limit**85th Percentile Speed
			70
			3
			210Upper Limit of the Pace
			60
			3
			180Average Test Run Speed
			60
			4
			240**Sum**
			**630**

The weighted average is 630/10 = 63 km/h, which suggests a speed limit of 60 km/h.

The suggested speed limit needs to be checked against the major physical features of the road using Table 19.

**Design Speed (km/h)**
			**Average Distance Between Intersections (m)**
			**Length of Proposed Zone (km)**
			**Maximum Speed Limit (km/h)**110
			400
			1.5
			110100
			300
			1.0
			10090
			250
			0.8
			9090
			175
			0.7
			8070
			125
			0.6
			7070
			100
			0.5
			6050
			75
			0.4
			5050
			60
			0.3
			4030
			45
			0.2
			30

All three criteria are satisfied with the 60 km/h speed limit. Therefore the minimum study recommends a speed limit of 60 km/h.

For the detailed analysis, the following data is collected:

Functional classification - Arterial
	Number of non-commercial driveways - 10/km
	Number of commercial driveways - 4.5/km
	Lane width - 3.65 m
	4.3 m painted median
	No shoulders, and barrier curbs on both sides of the road
	Light pedestrian activity for age < 12
	A sidewalk with a 0.3 meter setback
	No parking allowed
	Rolling terrain
	Crash rate = 7.4 crashes per million-vehicle-kilometers (average is 5.1 crashes per million-vehicle-kilometers for arterial roads)The factors determined from the various tables are:

**Adjustment Factors**Non-commercial Access (Table 20)
			+5Commercial Access (Table 20)
			0Lane Width (Table 21)
			+ 10Functional Classification (Table 22)
			0Median Type (Table 23)
			0Shoulder Type and Width (Table 24)
			
			-5Pedestrian Activity (Table 25)
			
			-10Parking Activity (Table 26)
			+ 15Roadway Alignment (Table 27)
			+5Crash Rate (Table 28)
			
			-10**Totals**
			**+35**
			**-25**
			**=+10**

The overall adjustment factor (OAF) is +10, which can be used to determine the multiplication factor as:

MF = (100+OAF)/100 = (100+10)/100 = 1.10

The multiplication factor is less than 1.25 and greater than 0.75, therefore the recommended speed limit is the speed limit from the minimum study multiplied by the multiplication factor and rounded to the nearest 10 km/h:

SL = 60 km/h * 1.10 = 66 km/h - 70 km/h
      
  Book traversal links for Appendix D: Northwestern Speed Zoning Technique
      ‹ Appendix C: Illinois Policy on Setting Speed Limits
                    Up
                    Appendix E: Speed Limits New Zealand (Road Risk Methodology) ›
                
                Book:Methods and Practices for Setting Speed Limits: An Informational Report
              Background
                Purpose & Scope
                Report Organization
                The Safety of Speed
                Speed Limit Basics
                Speed Limit Sign and Placement
                Speed Study Data Collection
                Speed Limit Enforcement
                Case Studies
                Summary of Results
                References
                Appendix A: Glossary
                Appendix B: Example Traffic Control Order
                Appendix C: Illinois Policy on Setting Speed Limits
                Appendix D: Northwestern Speed Zoning Technique
                Appendix E: Speed Limits New Zealand (Road Risk Methodology) U.S. DEPARTMENT OF TRANSPORTATION

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