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, selfselected 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.
(Extended Text Description: Equation: SL = (3SL_{85}+3SL_{pace}+4SL_{run}) / 10)
Where: SL = Weighted average speed limit
SL_{85} = Speed limit justified by the 85th percentile speed using Table 18
SL_{pace} = Speed limit justified by the upper limit of the 15 km/h pace using Table 18
SL_{run} = 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:
(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 noncommercial 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) 

NonCommercial 
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 Areawide 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 noncommercial 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 millionvehiclekilometers (average is 5.1 crashes per millionvehiclekilometers for arterial roads)
The factors determined from the various tables are:
Adjustment Factors 




Noncommercial 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