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                        Methods and Practices for Setting Speed Limits: An Informational Report


            
            In This Section
        Speed Management Safety
                USLIMITS2
                                Overview
                Create a New Project
                Revise an Existing Project
                Frequently Asked Questions
                Technical Support
                Facts & Statistics
                Engineering Speed Limits
                Traffic Calming ePrimer
                Ongoing Research
                Reference Materials
                Related Web Site Links
                
                Case Studies
To demonstrate the application of some of the principles and methods presented in this informational report, two case studies are presented. The case studies use existing roads and real data. In both cases, the posted speed limit is determined by the engineering, expert system, optimal speed, and safe systems methods. It is noted that not all of the data was collected for each of the methods, and for the sake of presenting each of the speed limit setting methods, reasonable assumptions were made about some values.

The outcomes may or may not match with the actual posted speed limit as determined by the governing road authority. This does not suggest that the road authorities are using outdated or incorrect methods in setting their speed limits. On the contrary, the methods used to set the initial and/or revised speed limits were in compliance with State statutes and requirements and the guidance provided in the

federal MUTCD.

It must be remembered that in all speed limit setting studies, the tools and techniques that are available to the practitioner are intended to assist the practitioner in making a decision—it is guidance and not direction as to the speed limit to be posted. Engineering judgment must be applied.



**CASE STUDY 1: Urban Collector Road**

As part of a speed limit reevaluation process, the City of Palm Bay, Florida, identified Eldron Boulevard from Jupiter Boulevard to Raleigh Road for analysis. The study area was selected based on the following considerations:

The roadside development is consistent throughout the study area;
	The physical features of the road are consistent throughout the study area; and
	The study area is bounded by signalized intersections at both ends of Eldron Boulevard, which form a natural break-point for speed zoning.Eldron Boulevard is a north-south collector road approximately 2.3 miles long and located in a northeastern portion of the City. The study segment is essentially straight and flat, with a design speed of 50 mph or more. It is a two-lane, undivided facility with no shoulder and an 8-foot wide sidewalk on the east side of the road, located about eight feet from the edge of the pavement. The lane widths are 12 feet each.

The contiguous and surrounding land use is single-family residential. There are 85 single-family residential driveways, one minor commercial driveway, and 26 two-way, stop controlled intersections that access Eldron Boulevard. The area is basically fully developed, and traffic volumes are relatively stable over time.

Eldron Boulevard is a municipal transit route. Parking is not prohibited, but happens very infrequently. Pedestrian volumes are typical for a residential street, and cyclist volumes are considered low. Street lighting is present throughout the study area.

Given the consistent physical features throughout the study area, a single spot speed measurement location was deemed sufficient. However, given the length of the study area, and as an extra precaution, the analysts decided to measure spot speeds at two locations. The spot speed stations were spaced evenly in the study area, and were located away from intersections or major driveways that would include vehicles changing speeds while using these accesses. Pneumatic road tubes were used to collect spot speed data at two locations in the study area; the results are as follows:

**North Station**
			**South Station**Posted Speed Limit
			40
			40Median Speed (50th Percentile)
			38.8
			37.2Average (Mean) Speed
			39.3
			32.285th Percentile Speed
			43.4
			43.010-mph Pace
			35 - 44
			35 - 44Percent Exceeding the Speed Limit
			48.3
			32.2Five test runs were undertaken through the study area, and the average test run speed was 41 mph.

From January 2009 to July 2010 (19 months), there were 19 crashes in the study area. Two of these crashes resulted in personal injuries, none in fatalities. The average daily traffic volume during this time was 9200 vehicles per day. The crash rate for this road is 1.55 crashes per million-vehicle-miles (MVM). The average crash rate for these types of facilities is 2.22 crashes/MVM.



**Engineering Method Using Operating Speed****Using the Illinois DOT Method**

**STEP 1:****Establish the Prevailing Speed**

The prevailing speed is the average of the 85th percentile speed, the upper limit of the 10 mph pace, and the average test run speed, rounded to the nearest 5 mph increment.

**A**
			**B**
			**C**
			**(A+B+C)/3**Station
			85th Percentile Speed (mph)
			Upper Limit of the 10-mph Pace (mph)
			Average Test Run Speed (mph)
			Prevailing Speed (mph)North
			43.4
			44
			41
			42.8South
			43.0
			44
			41
			42.7The prevailing speed rounded to the nearest 5 mph increment is 45 mph for both locations in the study area.



**STEP 2:****Supplementary Investigations**

Adjustment factors for determining the proposed posted speed limit as determined by further investigation of the following four conditions:

Elevated Crash Risk: The speed zone being studied has a crash rate of 1.55 crashes/MVM, which is lower than the statewide average of 2.22 crashes/MVM for these types of roads. Hence, there is no adjustment required for crash risk.
	Access Control: The access conflict number (ACN) is calculated for the speed zone, based on 85 single-family, residential driveways, one minor, commercial driveway, and 27 two-way, stop-controlled intersections in the study area:**ACN***
			**Reduction (%)**< 40
			041 to 60
			5> 60
			10Please see Extended Text Description below.

(*Extended Text Description:* Equation: **ACN* = (*Ns*+5*Nm*+10*Ni*) / *L* )

Where:

*Ns*= Number of field entrances and driveways to single-family dwellings

*Nm*= Number of driveways to minor commercial entrances, multi-family residential units, and minor street intersections

*Ni*= Number of driveways to major commercial entrances, large multi-family developments, and major street intersections

Therefore, based on accesses, it is appropriate to lower the prevailing speed (from Step 1) by 10 percent.

Pedestrian Activity: The pedestrian activity is typical for a residential area, is accommodated by a sidewalk on one side of the street, and is not considered "significant pedestrian activity." No further adjustment is required for this factor.
	Parking: Parking is negligible and is not a factor in determined the posted speed limit. The total adjustment from the 4 different factors is 10 percent.

**Step 3:****Selection of Preliminary Speed Limit**

The preliminary speed limit is either the calculated prevailing speed (from Step 1), or if the optional investigation was undertaken, it is the prevailing speed as adjusted by application of the percentage corrections from the optional investigation (Step 2). Since Step 2 was undertaken, the preliminary posted speed limit is:

45 mph - (0.1*45 mph) = 41 mphThe following rules apply to the outcome:

The preliminary posted speed limit should be the closest 5 mph increment to the (adjusted) prevailing speed. This results in a preliminary posted speed of 40 mph.
	The preliminary posted speed limit shall not differ from the prevailing speed (from Step 1) by more than 9 mph or by more than 20 percent, whichever is less. This condition is satisfied by the 40 mph preliminary posted speed limit.Therefore, the proposed preliminary speed limit is 40 mph.



**Step 4:****Violation Check**

The proposed speed limit should be either the preliminary posted speed limit or the 50th percentile speed, whichever is greater. In this case, the median speeds are 38 to 39 mph, so the preliminary posted speed limit of 40 mph is valid.

It is noted that the statutory speed limit for Eldron Boulevard is 30 mph, which is less than the preliminary posted speed limit determined above. At this point, the road authority has the option of either posting at the statutory speed, or the proposed speed limit.

The Illinois method of setting speed limits results in a recommended speed limit of 40 mph.



**Using the Northwestern Speed Zoning Technique**

**Step 1:****The Minimum Speed Study**

**Station**
			**85th Percentile Speed, km/h (mph)**
			**Upper Limit of the 10-mph Pace, km/h (mph)**
			**Average Test Run Speed, km/h (mph)**North
			43.4 (70)
			71 (44)
			66 (41)South
			43.0 (69)
			71 (44)
			66 (41)For the minimum speed study, 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
			70
			3
			210Average Test Run Speed
			80
			4
			320Sum
			740The weighted average is 740/10 = 74 km/h, which suggests a speed limit of 75 km/h based solely on the speed data.

The suggested speed limit needs to be checked against the major physical features of the road. The design speed of Eldron Boulevard is 50 mph (80 km/h), and the length of road under study is 2.3 miles (3.7 kms). The average distance between intersections is:

Please see Extended Text Description below.

(*Extended Text Description:* Equation: *D*=(2.3 *miles*/26 *intersections*) * (5280 *feet*/*mile*) = 467 *feet* = 142 *meters*)

**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
			30All three criteria are satisfied by a 70 km/h (45 mph) speed limit. Therefore the minimum study recommends a speed limit of 70 km/h (45 mph).

Continuing on with the detailed analysis, the factors determined from the various tables are:

**Adjustment Factors**Non-commercial Access (Table 20)
			
			-5Commercial Access (Table 20)
			+5Lane Width (Table 21)
			+5Functional Classification (Table 22)
			
			-5Median Type (Table 23)
			0Shoulder Type and Width (Table 24)
			0Pedestrian Activity (Table 25)
			
			-15Parking Activity (Table 26)
			0Roadway Alignment (Table 27)
			+ 10Crash Rate (Table 28)
			+ 10**Totals**
			**+30**
			**-25**
			**=+5**The overall adjustment factor (OAF) is +5 which can be used to determine the multiplication factor (MF) as:

MF = (100+OAF)/100 = (100+5)/100 = 1.05

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

SL = 70 km/h * 1.05 = 73.5 km/h - 75 km/h or 45 mph

The recommended speed limit based on the Northwestern Speed Zoning method is 45 mph.

**Expert Systems Approach Using USLIMITS2**

The data from the Eldron Boulevard speed limit study was entered into the USLIMITS2 program to determine the recommended speed limit for this section of road. The entered data and the recommended speed limit are shown in the boxed area below. The speed limit recommended by the USLIMITS2 approach is 40 mph.



**USLIMITS2 Data Output**

			Top of Form**Basic Project Information**

			**Project Name**- **Case Study 1**
			Project Number -
			Project Date - 09-21-2011
			State - Florida
			County - Brevard County
			City - Palm Bay City
			Route - Eldron Boulevard
			Route Type - Road Section in Developed Area
			Termini from - Jupiter Boulevard
			Termini to - Raleigh Road
			Route Status - EXISTING
			Description - FHWA/ITE Informational Report
			Case Study

			**Roadway Information**
			85th Percentile Speed - 43 mph
			50th Percentile Speed - 39 mph
			Section Length - 2.30 mile(s)
			Statutory Speed Limit - 30 mile(s)
			AADT - 9200
			Adverse Alignment - No
			Lanes and Presence/Type of Median - Two-lane road or undivided multi-lane.
			Number of Lanes - 2
			Area Type - Residential Collector
			Number of Driveways - 112
			Number of Signals - 0
			On Street Parking and Usage - Not High
			Pedestrian / Bicyclist Activity - High

			**Crash Data Information**
			Crash Data Months/Years - 1.58
			Crash AADT - 9200
			Total Number of Crashes - 19
			Total Number of Injury Crashes - 2
			Section Crash Rate - 155
			Section Injury Rate - 16
			Crash Rate Average for Similar Sections - 222
			Injury Rate Average for Similar Sections - 73
			Comments -

			**Recommended Speed Limit is: 40**

			**Note:****The final recommended speed limit is higher than the statutory speed limit for this type of road. The statutory limit is 30 mph.****Optimal Speed Limit**

The optimal speed limit is determined by calculating and selecting the speed that produces the lowest societal cost. In this case study only crash costs and fuel costs will be considered to demonstrate the method. In a full analysis, other societal costs would be analyzed, including time travel costs, automobile emissions, etc.

**Step 1:****Calculate the Crash Costs**

The road authority has developed the following crash prediction models using regression techniques, traffic, infrastructure, and historic crash data for roads under their control:

Please see Extended Text Description below.

(*Extended Text Description:* Equation: *NP+B* = *EXP*(2.75 - 0.089*SL* - 0.815*UMA* + ( (43.8 * *ADT* * *L*)/106) ) )

Please see Extended Text Description below.

(*Extended Text Description:* Equation: *Nv-v* = *EXP*(0.95 + 0.13*X* + 0.71*Xs* + 0.000014*ADT* - 0.026*SL* - 0.0069*W* + 0.19*NOL* - 0.38*GM* - 0.42*UMA* - 1.19*UC* - 2.5*UL*))

Where:

NP+B = Number of pedestrian and bicyclist crashes per year, per mile

*Nv-v*= Number of vehicle-vehicle crashes per year, per mile

*SL*= Posted Speed Limit (mph)

*X*= Number of intersections in the segment

*Xs*= Number of signalized intersections on the segment

*W*= Pavement width (feet)

*NOL*= Number of lanes

*GM*= 1 if median, 0 if no median

*UMA*= 1 if urban minor arterial, 0 if not

*UC*= 1 if urban collector street, 0 if not

*UL*= 1 if urban local road, 0 if not

*ADT*= Average daily traffic

*L*= Length (miles)

Additionally, the road authority has examined its severity distributions of the two crashes types based on speed, and has produced the following probabilities using the KABCO severity scale. The KABCO severity scale was developed by the National Safety Council, and is used by the investigating officers to classify injury severity for occupants into one of five categories: K - killed; A - disabling injury; B - evident injury; C - possible injury; O - no apparent injury. These definitions may vary slightly for different police agencies.

**Probability of Crash Severity for Vehicle-Pedestrian/Cyclist Crashes**

**Speed Limit (mph)**
			**Crash Severity****K**
			**A**
			**B**
			**C**
			**O**20
			0.0028
			0.0339
			0.2053
			0.5631
			0.194925
			0.0040
			0.0435
			0.2335
			0.5555
			0.163530
			0.0057
			0.0549
			0.2622
			0.5415
			0.135735
			0.0080
			0.0684
			0.2905
			0.5219
			0.111240
			0.0110
			0.0841
			0.3178
			0.4970
			0.090145
			0.0150
			0.1020
			0.3432
			0.4677
			0.072150
			0.0202
			0.1221
			0.3657
			0.4349
			0.057155
			0.0268
			0.1443
			0.3846
			0.3997
			0.044660
			0.0351
			0.1682
			0.3993
			0.3630
			0.0344

**Probability of Crash Severity for Vehicle-Pedestrian/Cyclist Crashes**

**Speed Limit (mph)**
			**Crash Severity****K**
			**A**
			**B**
			**C**
			**O**20
			0.002
			0.0006
			0.0081
			0.0862
			0.904925
			0.0004
			0.0009
			0.0116
			0.1081
			0.87930
			0.0006
			0.0015
			0.0158
			0.1313
			0.850835
			0.001
			0.0022
			0.0218
			0.1591
			0.815940
			0.0016
			0.0031
			0.0289
			0.187
			0.779445
			0.0025
			0.0044
			0.0386
			0.2188
			0.735750
			0.0037
			0.0062
			0.0495
			0.2491
			0.691555
			0.0055
			0.0088
			0.0635
			0.2816
			0.640660
			0.008
			0.0117
			0.0788
			0.3105
			0.591

The City of Palm Bay uses the societal costs of crashes shown below:

**Crash Severity**
			**Societal Cost ($)**K
			3,366,388A
			233,100B
			46,620C
			24,510O
			2,590

Therefore, employing the crash model for vehicle-pedestrian/cyclist, the probability distributions for the different crash severities, and the societal costs for the different crash severities, the cost of vehicle-pedestrian/cyclist crashes for the different available speed limits is shown below.

**Speed Limit (mph)**
			**No. of Ped/****Cyclist****Crashes**
			**Crash Costs by Severity ($)****K**
			**A**
			**B**
			**C**
			**O**
			**Total**20
			6.7
			62,820
			52,664
			63,787
			91,982
			3,364
			274,61725
			4.3
			57,509
			43,306
			46,491
			58,149
			1,809
			207,26330
			2.7
			52,516
			35,024
			33,455
			36,324
			962
			158,28035
			1.8
			47,233
			27,963
			23,752
			22,435
			505
			121,88840
			1.1
			41,618
			22,033
			16,652
			13,691
			262
			94,25645
			0.7
			36,368
			17,124
			11,524
			8,256
			134
			73,40750
			0.5
			31,385
			13,136
			7,869
			4,920
			68
			57,37855
			0.3
			26,684
			9,948
			5,303
			2,897
			34
			44,86760
			0.2
			22,395
			7,431
			3,528
			1,686
			17
			35,058

It is noted that under this particular crash model, the number of pedestrian and cyclist crashes decreases as the speed limit increases. This is likely due to the fact that higher speed roads have lower pedestrian and cyclist traffic than similar lower speed roads. In other words, exposure to these types of crashes decreases as speed increases.

The same methodology is employed to identify the societal cost of vehicle-vehicle crashes for the different speed limit alternatives.

**Speed Limit (mph)**
			**No. of Vehicle-Vehicle Crashes**
			**Crash Costs by Severity ($)****K**
			**A**
			**B**
			**C**
			**O**
			**Total**20
			19.4
			—
			2,256
			5,145
			32,599
			46,386
			86,38625
			17.0
			114,456
			2,378
			6,419
			35,916
			39,842
			199,01130
			14.9
			20,101
			3,132
			8,073
			39,551
			33,984
			104,83935
			13.1
			26,475
			4,583
			9,655
			42,183
			28,884
			111,78040
			11.5
			38,746
			5,902
			11,698
			44,883
			24,322
			125,55245
			10.1
			54,437
			7,303
			13,617
			46,323
			20,402
			142,08250
			8.9
			74,689
			9,102
			15,970
			47,593
			16,910
			164,26555
			7.8
			97,064
			11,262
			17,983
			47,578
			13,957
			187,84560
			6.8
			126,696
			14,037
			20,257
			47,229
			11,353
			219,572

It is noted that the number of crashes decreases as the speed limit increases. This is likely due to the fact that the design of higher speed roads affords more generous dimensions and greater safety features than similar lower speed facilities.

The total crash costs are the sums of the vehicle-vehicle crash costs and the vehicle-pedestrian/cyclist crash costs.

**Speed Limit (mph)**
			**Ped/Cyclist Crash Costs ($)**
			**Vehicle-Vehicle Crash Costs ($)**
			**Total Crash Costs ($)**20
			274,617
			86,386
			361,00325
			207,263
			199,011
			406,27530
			158,280
			104,839
			263,12035
			121,888
			111,780
			233,66940
			94,256
			125,552
			219,80845
			73,407
			142,082
			215,48950
			57,378
			164,265
			221,64255
			44,867
			187,845
			232,71260
			35,058
			219,572
			254,630

According to published data on the fuel efficiency of late model passenger cars and light trucks according to speed, the annual fuel consumption for different speed limits can be calculated. Assuming a cost for gas of $3.50/gallon yields an annual fuel cost. This annual fuel cost can be added to the annual crash costs to determine the net societal cost for the different speed limit alternatives. The results are shown below.

**Annual Fuel****Speed Limit** **(mph)**
			**Fuel Economy****(mpg)**
			**Gallons**
			**Fuel Cost** **(at $3.50/gal)**
			**Annual Crash****Cost ($)**
			**Total Cost ($)**20
			26.4
			292,553
			1,023,936
			361,003
			1,384,93925
			30.2
			255,848
			895,467
			406,275
			1,301,74130
			31.8
			243,161
			851,063
			263,120
			1,114,18335
			32.7
			236,190
			826,664
			233,669
			1,060,33240
			32.6
			236,733
			828,564
			219,808
			1,048,37245
			32.8
			235,559
			824,457
			215,489
			1,039,94650
			32.1
			240,698
			842,443
			221,642
			1,064,08655
			31.1
			248,441
			869,542
			232,712
			1,102,25460
			29.0
			266,095
			931,332
			254,630
			1,185,961

Under an optimal speed limit approach to Eldron Boulevard, the recommended speed limit is 45 mph. The subsequent analysis does not include the cost of travel time, or automobile emissions, which would typically be included in the thorough analysis.



**Safe Systems Approach**

Under the safe systems approach to setting the speed limit, the critical factor is the type of crashes that can be expected to occur given the physical features of the road, and the types of road users that are expected to be encountered.

In this instance, pedestrians and cyclists are permitted to use Eldron Boulevard but cyclists are very infrequent, and pedestrians walking along the road are provided with a sidewalk that is set back from the edge of pavement. The fact is that pedestrian-vehicle and cyclist-vehicle interactions are uncommon, and are not a significant factor in determining a "safe speed limit."

However, there are 26 two-way, stop controlled intersections and 86 driveways (85 residential and one minor commercial) along this section of Eldron Boulevard. This means that the potential for right-angle crashes, which have the greatest potential for causing serious injury to vehicle occupants, is significant. Therefore, under a safe systems approach to setting speed limits, speeds on Eldron Boulevard from Jupiter Road to Raleigh Road should be limited to those speeds where a right-angle crash will not cause any serious injuries. This being the case, research on human biomechanical tolerance, and vehicle crashworthiness studies indicate that speeds should be limited to 30 mph.

The recommended speed limit under the safe systems approach is 30 mph.

The recommended speed limits yielded by each speed limit setting method, and the actual speed limit enacted by the road authority are shown in Tables 15 and 16.

**Table 15. Recommended Speed Limits for the Eldron Boulevard Case Study**

**Eldron Boulevard, Florida**Actual Speed Limit
			40Illinois DOT
			40Northwestern
			45USLIMITS2
			40Optimal Speed
			45Safe System Speed
			30

**CASE STUDY 2: Rural Arterial Road**

Roadway improvements undertaken in an 11.5-mile segment of State Route 67 between Milepost 11.3 and 22.8 in California prompted the need for a re-examination of the existing 55 mph speed limit. This is an existing road with consistent road and land use characteristics throughout.

State Route 67 is a 24.4-mile road running primarily in a north-south direction between the City of El Cajon and the community of Ramona. The study segment is a 2-lane highway, which traverses hilly terrain, resulting in undulating vertical grades and winding horizontal curves. Strategically-located passing lanes are present through the study area.

Traffic volumes are as follows:

**Milepost**
			**2011 AADT**13.56
			24,90015.20
			25,50020.87
			25,00021.35
			29,500Average
			25,350

The roadway is asphalt with varying shoulder widths throughout. Shoulders are generally paved and range from 3 feet (1.0 meter) to 8 feet (2.4 meters).

A painted median is provided that includes a median rumble strip and raised pavement markers. The median is typically three feet wide.

In addition to the appropriate traffic control devices and traffic barriers, 11 digital Speed Feedback signs are employed.

There are three signalized intersections in the study area.

A Doppler radar system was used to conduct spot speed studies at five different locations within the study area.

**Milepost**
			**85th Percentile Speed (mph)**
			**Median Speed (mph)**
			**Mean Speed (mph)**
			**Upper Limit of the 10 mph Pace (mph)**
			**Percent Exceeding the 55 mph**11.30
			61
			56
			56
			61
			5715.00
			63
			57
			57
			60
			6018.10
			65
			60
			60
			64
			8319.70
			59
			56
			56
			60
			5921.00
			54
			51
			51
			55
			8Average
			60.4
			56.0
			56.0
			60.0

Based on a review of the horizontal alignment in the study section, the design speed is 50 mph. None of the curves require an advisory speed warning.

The 3-year crash rate for the entire study area is 0.80 crashes per million-vehicle-miles, which is lower than the statewide average crash rate of 1.51 crashes per million-vehicle-miles. However, there have been 17 fatal crashes in the 3-year analysis period that have been evenly distributed throughout the study section. The fatal crash rate is 0.46 crashes per million-vehicle-miles (MVM), which is almost double the statewide average fatal crash rate of 0.25 crashes/MVM.

Public and private accesses to State Route 67 were inventoried and consist of the following:

**NBND (milepost)**
			**SBND (milepost)**Private Access
			12.10
			11.4912.17
			11.9612.38
			12.7812.76
			13.6913.80
			13.7813.96
			14.0614.07
			14.8914.10
			15.5614.34
			16.1914.36
			16.2814.89
			16.4515.46
			17.4315.67
			17.5115.89
			18.1616.13
			18.2716.96
			18.3717.00
			18.5417.89
			18.9218.0518.1418.2120.25Public Road
			14.35 (Iron Mountain Trail)
			17.73 (Rockhouse Road)
			20.87 (Mussey Grade Road)
			13.56 (Scripps Poway Parkway)
			15.15 (Poway Road)
			18.11 (Mount Woodson Road)
			18.55 (Archie Moore Road)All of the private accesses are to single family residential dwellings or small-scale agricultural operations.

State Route 67 has a rural cross-section, so there are no sidewalks. Pedestrians and cyclists are not prohibited from the facility, but the volume of both user groups is extremely low.

Parking is permitted, but there are no significant roadside attractions, so parking activity is negligible.

Five test runs were undertaken during off-peak periods, and the average test run speed was 63 km/h.



**Engineering Method Using Operating Speed**

**Using the Illinois DOT Method**

**STEP 1:****Establish the Prevailing Speed**

The prevailing speed is the average of the 85th percentile speed, the upper limit of the 10 mph pace, and the average test run speed, rounded to the nearest 5 mph increment.

**A**
			**B**
			**C**
			**(A+B+C)/3***Milepost*
			*85th Percentile Speed (mph)*
			*Upper Limit of the 10 mph Pace (mph)*
			*Avg. Test Run Speed (mph)*
			*Prevailing Speed*
			*Rounded*11.30
			61
			61
			63
			62
			6015.00
			63
			60
			63
			62
			6018.10
			65
			64
			63
			64
			6519.70
			59
			60
			63
			61
			6021.00
			54
			55
			63
			57
			55

**STEP 2:****Supplementary Investigations (Optional)**

Adjustment factors for determining the proposed posted speed limit may be determined by further investigation of any or all of the following four conditions:

Elevated Crash Risk: The speed zone being studied has a crash rate that is lower than the statewide average for similar facilities. However, it is still deemed a high-crash segment based on the elevated fatal crash rate, which is double the statewide average. Therefore, a 10 percent reduction in the prevailing speed is appropriate.
	Access Control: The access conflict number (ACN) is calculated for the speed zone, based on 40 private accesses to residential driveways and farms, and 7 public road intersections.**ACN***
			**Reduction (%)**< 40
			041 to 60
			5> 60
			10Please see Extended Text Description below.

(*Extended Text Description:* Equation: **ACN* = (40*1+7*5)/11.5 *miles* = 6.5)

Therefore, no adjustment is required for access concerns.

Pedestrian Activity: There is no significant pedestrian activity, so no further adjustment is required for this factor.
	Parking: Parking is negligible and is not a factor in determining the posted speed limit.The total adjustment from the 4 different factors is 10 percent.



**Step 3:****Selection of Preliminary Speed Limit**

The preliminary speed limit is either the calculated prevailing speed (from Step 1), or if the optional investigation was undertaken, it is the prevailing speed as adjusted by application of the percentage corrections from the optional investigation (Step 2).

**Milepost**
			**Prevailing Speed (Step 1), mph**
			**Adjusted Speed (Step 2), mph**
			**Preliminary Speed Limit (Rounded), mph**11.30
			60
			54
			5515.00
			60
			54
			5518.10
			65
			59
			6019.70
			60
			54
			**1**5521.00
			55
			50
			50The following rules apply to the outcome:

The preliminary posted speed limit should be the closest 5 mph increment to the (adjusted) prevailing speed.
	The preliminary posted speed limit shall not differ from the prevailing speed (from Step 1) by more than 9 mph or by more than 20 percent, whichever is less.Both of these conditions are satisfied by the preliminary speed limits reported above.



**Step 4:****Violation Check**

The proposed speed limit should be either the preliminary posted speed limit or the 50th percentile speed, whichever is greater. In all cases, the preliminary speed limit and the median speeds yield the same speed limit (rounded to the nearest 5 mph increment).

**Milepost**
			**Preliminary, mph**
			**Median, mph**11.30
			55
			5615.00
			55
			5718.10
			60
			6019.70
			55
			5621.00
			50
			51

If the proposed speed limit exceeds the statutory speed limit for the highway in question, either the statutory speed or the proposed speed limit may be posted. If the selected speed limit results in a violation rate greater than 50 percent, the appropriate police agency(ies) should be notified that extra enforcement efforts may be necessary.

It is noted that differences in posted speeds between adjacent speed zones should not be more than 10 mph. However, the Illinois policy permits a larger difference provided that adequate speed reduction signs are posted.

**Using the Northwestern Speed Zoning Technique**

**Milepost**
			**85th Percentile Speed (mph) (km/h)**
			**Upper Limit of the 10 mph Pace (mph) (km/h)**
			**Average Test Run Speed (mph) (km/h)**11.30
			61 (98)
			61 (98)
			63 (101)15.00
			63 (101)
			60 (97)
			63 (101)18.10
			65 (105)
			64 (103)
			63 (101)19.70
			59 (95)
			60 (97)
			63 (101)21.00
			54 (87)
			55 (88)
			63 (101)

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

**Justified from Table 13****Milepost**
			**85th Percentile Speed (mph) (km/h)**
			**Upper Limit of the 10 mph Pace (mph) (km/h)**
			**Average Test Run Speed (mph) (km/h)**
			**Weighted Limit (mph) (km/h)**
			**Speed Limit (Rounded), mph (km/h)**11.30
			60 (100)
			65 (110)
			65 (110)
			635 (1070)
			65 (110)15.00
			60 (100)
			65 (110)
			65 (110)
			635 (1070)
			65 (110)18.10
			65 (110)
			65 (110)
			65 (110)
			650 (1100)
			65 (110)19.70
			55 (90)
			65 (110)
			65 (110)
			620 (1040)
			60 (100)21.00
			55 (90)
			60 (100)
			65 (110)
			605 (1010)
			60 (100)

The recommended speed limit based on the minimum study is 60 to 65 mph depending on the location within the study area. The lower speeds being produced at the higher mileposts may cause the analyst to review the site and traffic conditions to determine if the speed zone should be divided into two separate zones. The conditions are consistent through the study area, and the difference in the speed limit recommended by the minimum study is only 5 mph. Therefore, the study area will be considered as one speed zone.

The speed limit from the minimum study needs to be checked against the major physical features of the road. The design speed of State Route 67 is 50 mph (80 km/h), and the length of the road under study is 11.5 miles (18.5 kms). The average distance between intersections is:

Please see Extended Text Description below.

(*Extended Text Description:* Equation: *D* = (11.5 *miles*/7 *intersections*) * (5280 *feet/mile*) = 8674 *feet* = 2643 *meters*)

**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 70 km/h speed limit. Therefore, the minimum study recommends a speed limit of 45 mph (70 km/h).

Continuing on with the detailed analysis, the factors determined from the various tables are:

**Adjustment Factors**Non-commercial Access (Table 20)
			
			-5Commercial Access (Table 20)
			+5Lane Width (Table 21)
			+5Functional Classification (Table 22)
			
			-5Median Type (Table 23)
			0Shoulder Type and Width (Table 24)
			0Pedestrian Activity (Table 25)
			
			-15Parking Activity (Table 26)
			0Roadway Alignment (Table 27)
			+ 10Crash Rate (Table 28)
			+ 10**Totals**
			**+30**
			**-25**
			**=+5**

The overall adjustment factor (OAF) is +30 which can be used to determine the multiplication factor (MF) as:

MF = (100+OAF)/100 = (100+30)/100 = 1.30

The multiplication factor is greater than the maximum allowed, so it is reduced to 1.25. Therefore, the recommended speed limit (SL) is the speed limit from the minimum study multiplied by the multiplication factor and rounded to the nearest 10 km/h:

SL = 70 km/h * 1.25 = 87.5 km/h - 90 km/h or 55 mph

The recommended speed limit based on the Northwestern Speed Zoning method is 55 mph.



**Expert System (USLIMITS2)**

The data from the State Route 67 speed limit study was entered into the USLIMITS2 program to determine the recommended speed limit for this section of road. The entered data and the recommended speed limit are shown in the boxed area below. The recommended speed limit is 55 mph.

The speed data at Mileposts 18.10 and 21.00, if entered into USLIMITS2 using the same traffic and geometric data as above, will produce recommended speed limits of 60 mph and 50 mph, respectively. These are only 5 mph different from the other mileposts, and it is desirable to use a consistent 55 mph throughout the study area to encourage speed limit compliance.



**Optimal Speed Limit**

The optimal speed limit is determined by calculating and selecting the speed that produces the lowest societal cost. In this case study, only crash and fuel costs will be considered, to demonstrate the method. In a full analysis, other societal costs would be analyzed, including time travel costs, automobile emissions, etc.

**Step 1:****Calculate the Crash Costs**

The road authority has developed the following crash prediction models using regression techniques and traffic, infrastructure, and historic crash data for roads under its control:

*N = L * EXP*(0.000016 + 0.0102*SL*+ 0.00045*X*+ 0.0000015*ADT)*

Where: *N*= Number of crashes per year, per mile

*SL*= Posted Speed Limit (mph)

*X*= Number of intersections on the segment

*ADT*= Average daily traffic

*L*= Length (miles)

Additionally, the road authority has examined the severity distributions of the two crash types based on speed, and has produced the following probabilities using the KABCO severity scale. The KABCO severity scale was developed by the National Safety Council, and is used by investigating officers to classify injury severity for occupants into one of five categories: K - killed; A - disabling injury; B - evident injury; C -possible injury; O - no apparent injury. These definitions may vary slightly for different police agencies.

**USLIMITS2 Data Output**

			Top of Form

			**Basic Project Information****Project Name**- **Case Study 2**

			Project Number -
			Project Date - 09-21-2011
			State - California
			County - San Diego County
			City -
			Route - State Route 67
			Route Type - Road Section in Undeveloped
			Area
			Termini from - Milepost 11.3
			Termini to - Milepost 22.8
			Route Status - EXISTING
			Description - FHWA/ITE Informational Report
			Case Study

			**Roadway Information**
			85th Percentile Speed - 61 mph
			50th Percentile Speed - 56 mph
			Section Length - 11.50 mile(s)
			Statutory Speed Limit - 55 mile(s)
			AADT - 23500
			Adverse Alignment - No
			Lanes and Presence/Type of Median - Two-lane road or undivided multi-lane
			Number of Lanes - 2
			Roadside Hazard Rating - 3

			**Crash Data Information**
			Crash Data Months/Years - 3.00
			Crash AADT - 25000
			Total Number of Crashes - 252
			Total Number of Injury Crashes - 145
			Section Crash Rate - 80
			Section Injury Rate - 46
			Crash Rate Average for Similar Sections - 151
			Injury Rate Average for Similar Sections - 25
			Comments -

			**Recommended Speed Limit is: 55**

			**The crash rate of the section is 80 per 100 MVMT. The average rate for similar sections is 151 per 100 MVMT, and the critical rate is 169 per 100 MVMT. The crash rate of this section is 47 percent lower than the average crash rate for similar sections. The rate of injury crashes for the section is 46 per 100 MVMT. The average rate for similar sections is 25 per 100 MVMT, and the critical rate is 32 per 100 MVMT. The rate of injury crashes for this section is 84 percent higher than the average rate for similar sections. A comprehensive crash study should be undertaken to identify engineering and traffic control deficiencies and appropriate corrective actions. The speed limit should only be reduced as a last measure after all other treatments have either been tried or ruled out.**

**Probability of Crash Severity for Different Speeds**

**Speed Limit (mph)**
			**Crash Severity****K**
			**A**
			**B**
			**C**
			**O**40
			0.0016
			0.0031
			0.0289
			0.187
			0.779445
			0.0025
			0.0044
			0.0386
			0.2188
			0.735750
			0.0037
			0.0062
			0.0495
			0.2491
			0.691555
			0.0055
			0.0088
			0.0635
			0.2816
			0.640660
			0.0080
			0.0117
			0.0788
			0.3105
			0.591065
			0.0116
			0.0160
			0.1001
			0.3390
			0.533370
			0.0200
			0.1250
			0.3651
			0.4349
			0.055075
			0.0350
			0.1500
			0.3846
			0.4010
			0.0294

The road authority uses the societal costs of crashes shown below:

**Crash Severity**
			**Societal Cost ($)**K
			3,366,388A
			233,100B
			46,620C
			24,510O
			2,590

Therefore, by employing the crash model, the probability distributions for the different crash severities, and the societal costs for the different crash severities, the cost of crashes for the different available speed limits is shown below.

**Speed****Limit** **(mph)**
			**No. of Crashes**
			**Crash Costs by Severity ($)****K**
			**A**
			**B**
			**C**
			**O**
			**Total**40
			18.1
			97,627
			13,098
			24,421
			83,075
			36,589
			254,81045
			19.1
			160,524
			19,563
			34,324
			102,288
			36,344
			353,04450
			20.1
			250,006
			29,008
			46,319
			122,547
			35,948
			483,82955
			21.1
			391,076
			43,327
			62,529
			145,784
			35,045
			677,76060
			22.2
			598,601
			60,619
			81,655
			169,156
			34,023
			944,05465
			23.4
			913,386
			87,236
			109,154
			194,346
			32,308
			1,336,42970
			24.6
			1,657,202
			717,189
			418,953
			262,370
			3,506
			3,059,21975
			25.9
			3,051,845
			905,657
			464,421
			254,576
			1,972
			4,678,471

It is noted that the number of crashes increases as the speed limit increases. This is as expected.

According to published data on the fuel efficiency of late model passenger cars and light trucks according to speed, the annual fuel consumption for different speed limits can be calculated. An annual fuel cost can be calculated assuming a gasoline cost of $3.50/gallon. This annual fuel cost can be added to the annual crash costs to determine the net societal cost for the different speed limit alternatives. The results are shown below.

**Speed Limit (mph)**
			**Fuel Economy (mpg)**
			**Annual Fuel**
			**Annual Crash Cost ($)**
			**Total Cost ($)****Gallons**
			**Fuel Cost (at $3.50/gal)**40
			28.6
			3,448,995
			1,2071,482
			254,810
			12,326,29145
			29.2
			3,378,125
			11,823,438
			353,044
			12,176,481***50***
			***30.9***
			***3,192,273***
			***11,172,957***
			***483,829***
			***11,656,786***55
			31.1
			3,173,020
			11,105,569
			677,760
			11,783,32960
			29.0
			3,398,493
			11,894,724
			944,054
			12,838,77865
			26.5
			3,720,556
			13,021,947
			1,336,429
			14,358,37670
			24.1
			4,090,876
			14,318,066
			3,059,219
			17,377,28675
			21.8
			4,524,828
			15,836,898
			4,678,471
			20,515,369

Under an optimal speed limit approach to State Route 67, the recommended speed limit is 50 mph. The preceding analysis does not include the cost of travel time or automobile emissions, which would typically be included in the thorough analysis and may affect the outcome of the analysis.

**Safe Systems Approach**

Under the safe systems approach to setting the speed limit, the critical factor is the type of crashes that can be expected to occur given the physical features of the road, and the types of road users that are expected to be encountered. Pedestrian and cyclists, while permitted on State Route 67, are infrequent. Pedestrian-vehicle and cyclist-vehicle conflicts are rare, and do not factor into setting a speed limit using the safe systems approach.

The controlling criteria in this instance are the presence of at-grade intersections and driveways (which permit right-angle crashes), and the undivided cross-section (which permits head-on crashes). State Route 67 has several at-grade intersections that are two-way stop controlled. The volumes on these intersections are generally low and do not result in a significant right-angle crash risk because of extremely low exposure. This being the case, the most significant crash type is the head-on crash. Therefore, the appropriate speed limit under a safe system approach is about 50 mph (80 km/h).

The recommended speed limits yielded by each speed limit setting method and the actual speed limit enacted by the road authority for State Route 67 are shown in Table 16.

**Table 16. Recommended Speed Limits for the State Route 67 Case Study**

**State Route 67, California**Actual Speed Limit
			55Illinois DOT
			55Northwestern
			55USLIMITS2
			55Optimal Speed
			50Safe System Speed
			50
      
  Book traversal links for Case Studies
      ‹ Speed Limit Enforcement
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                    Summary of Results ›
                
                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

**Federal Highway Administration**

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WASHINGTON, DC 20590

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