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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 = (3SL85+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

30

34 - 44

33 - 42

30 - 38

40

45 - 54

43 - 52

39 - 48

50

55 - 64

53 - 62

49 - 56

60

65 - 74

63 - 72

57 - 65

70

75 - 84

73 - 80

66 - 75

80

85 - 94

81 - 88

76 - 85

90

95 - 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

110

100

300

1.0

100

90

250

0.8

90

90

175

0.7

80

70

125

0.6

70

70

100

0.5

60

50

75

0.4

50

50

60

0.3

40

30

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

0

4 - 6

0

+ 10

+ 10

+ 10

+5

+5

0

0

0

-5

7 - 12

1

+ 10

+ 10

+5

+5

0

0

0

-5

-5

13 - 21

2 - 3

+5

+5

0

0

0

-5

-5

-10

-10

22 - 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

-15

2.8 - 3.2

+5

+5

0

0

0

-5

-5

-5

-10

3.3 - 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

-20

Collector

+5

0

0

0

-5

-5

-10

-10

-15

Arterial

+ 10

+5

+5

0

0

0

-5

-5

-10

Expressway

+ 15

+ 10

+ 10

+5

0

0

0

0

-5

Freeway

+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

+20

Freeway

-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

+20

Collector

0

0

+5

+ 10

Arterial

-5

0

0

+5

Expressway

-10

-5

0

0

Freeway

-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

-5

Medium

-20

-15

-10

-5

0

Light

-15

-10

-5

0

0

If none, consider ages over 12

Age > 12

Heavy

-10

-5

0

0

0

Medium

-5

0

0

0

0

Light

-5

0

0

0

0

None

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

-10

Collector

+ 10

0

-10

-15

Arterial

+ 15

0

-10

-15

Expressway

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

0

1

0

0

-5

-5

2

-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%

+ 10

76% - 125%

0

126% - 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

210

Upper Limit of the Pace

60

3

180

Average 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

110

100

300

1.0

100

90

250

0.8

90

90

175

0.7

80

70

125

0.6

70

70

100

0.5

60

50

75

0.4

50

50

60

0.3

40

30

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)

+5

 

 

Commercial Access (Table 20)

0

 

 

Lane Width (Table 21)

+ 10

 

 

Functional Classification (Table 22)

0

 

 

Median Type (Table 23)

0

 

 

Shoulder Type and Width (Table 24)

 

-5

 

Pedestrian Activity (Table 25)

 

-10

 

Parking Activity (Table 26)

+ 15

 

 

Roadway Alignment (Table 27)

+5

 

 

Crash 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