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 This does not suggest that the road authorities are using outdated or incorrect methods in setting their speed limits.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 |
40 |
|
Median Speed (50th Percentile) |
38.8 |
37.2 |
|
Average (Mean) Speed |
39.3 |
32.2 |
|
85th Percentile Speed |
43.4 |
43.0 |
|
10-mph Pace |
35 - 44 |
35 - 44 |
|
Percent Exceeding the Speed Limit |
48.3 |
32.2 |
Five 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 |
42.8 |
|
South |
43.0 |
44 |
41 |
42.7 |
The 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 |
0 |
|
41 to 60 |
5 |
|
> 60 |
10 |
(Extended Text Description: Equation: *ACN = (Ns+5Nm+10Ni) / 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 mph
The 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 |
210 |
|
Upper Limit of the Pace |
70 |
3 |
210 |
|
Average Test Run Speed |
80 |
4 |
320 |
|
|
Sum |
740 |
|
The 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:
(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 |
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 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) |
|
-5 |
|
|
Commercial Access (Table 20) |
+5 |
|
|
|
Lane Width (Table 21) |
+5 |
|
|
|
Functional Classification (Table 22) |
|
-5 |
|
|
Median Type (Table 23) |
0 |
|
|
|
Shoulder Type and Width (Table 24) |
0 |
|
|
|
Pedestrian Activity (Table 25) |
|
-15 |
|
|
Parking Activity (Table 26) |
0 |
|
|
|
Roadway Alignment (Table 27) |
+ 10 |
|
|
|
Crash 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 Roadway Information Crash Data Information |
Comments - Recommended Speed Limit is: 40 Note: |
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:
(Extended Text Description: Equation: NP+B = EXP(2.75 - 0.089SL - 0.815UMA + ( (43.8 * ADT * L)/106) ) )
(Extended Text Description: Equation: Nv-v = EXP(0.95 + 0.13X + 0.71Xs + 0.000014ADT - 0.026SL - 0.0069W + 0.19NOL - 0.38GM - 0.42UMA - 1.19UC - 2.5UL))
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 |
0.1949 |
|
25 |
0.0040 |
0.0435 |
0.2335 |
0.5555 |
0.1635 |
|
30 |
0.0057 |
0.0549 |
0.2622 |
0.5415 |
0.1357 |
|
35 |
0.0080 |
0.0684 |
0.2905 |
0.5219 |
0.1112 |
|
40 |
0.0110 |
0.0841 |
0.3178 |
0.4970 |
0.0901 |
|
45 |
0.0150 |
0.1020 |
0.3432 |
0.4677 |
0.0721 |
|
50 |
0.0202 |
0.1221 |
0.3657 |
0.4349 |
0.0571 |
|
55 |
0.0268 |
0.1443 |
0.3846 |
0.3997 |
0.0446 |
|
60 |
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 |
0.9049 |
|
25 |
0.0004 |
0.0009 |
0.0116 |
0.1081 |
0.879 |
|
30 |
0.0006 |
0.0015 |
0.0158 |
0.1313 |
0.8508 |
|
35 |
0.001 |
0.0022 |
0.0218 |
0.1591 |
0.8159 |
|
40 |
0.0016 |
0.0031 |
0.0289 |
0.187 |
0.7794 |
|
45 |
0.0025 |
0.0044 |
0.0386 |
0.2188 |
0.7357 |
|
50 |
0.0037 |
0.0062 |
0.0495 |
0.2491 |
0.6915 |
|
55 |
0.0055 |
0.0088 |
0.0635 |
0.2816 |
0.6406 |
|
60 |
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 ($) |
|---|---|
|
K |
3,366,388 |
|
A |
233,100 |
|
B |
46,620 |
|
C |
24,510 |
|
O |
2,590 |
Therefore, employing the crash model for vehicle-pedestrian/cyclist, the probability distributions for the different crash severities, and the societal costs for the different crash severities, the cost of vehicle-pedestrian/cyclist crashes for the different available speed limits is shown below.
|
Speed Limit (mph) |
No. of Ped/ Cyclist Crashes |
Crash Costs by Severity ($) |
|||||
|---|---|---|---|---|---|---|---|
|
K |
A |
B |
C |
O |
Total |
||
|
20 |
6.7 |
62,820 |
52,664 |
63,787 |
91,982 |
3,364 |
274,617 |
|
25 |
4.3 |
57,509 |
43,306 |
46,491 |
58,149 |
1,809 |
207,263 |
|
30 |
2.7 |
52,516 |
35,024 |
33,455 |
36,324 |
962 |
158,280 |
|
35 |
1.8 |
47,233 |
27,963 |
23,752 |
22,435 |
505 |
121,888 |
|
40 |
1.1 |
41,618 |
22,033 |
16,652 |
13,691 |
262 |
94,256 |
|
45 |
0.7 |
36,368 |
17,124 |
11,524 |
8,256 |
134 |
73,407 |
|
50 |
0.5 |
31,385 |
13,136 |
7,869 |
4,920 |
68 |
57,378 |
|
55 |
0.3 |
26,684 |
9,948 |
5,303 |
2,897 |
34 |
44,867 |
|
60 |
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 ($) |
|||||
|---|---|---|---|---|---|---|---|
|
K |
A |
B |
C |
O |
Total |
||
|
20 |
19.4 |
— |
2,256 |
5,145 |
32,599 |
46,386 |
86,386 |
|
25 |
17.0 |
114,456 |
2,378 |
6,419 |
35,916 |
39,842 |
199,011 |
|
30 |
14.9 |
20,101 |
3,132 |
8,073 |
39,551 |
33,984 |
104,839 |
|
35 |
13.1 |
26,475 |
4,583 |
9,655 |
42,183 |
28,884 |
111,780 |
|
40 |
11.5 |
38,746 |
5,902 |
11,698 |
44,883 |
24,322 |
125,552 |
|
45 |
10.1 |
54,437 |
7,303 |
13,617 |
46,323 |
20,402 |
142,082 |
|
50 |
8.9 |
74,689 |
9,102 |
15,970 |
47,593 |
16,910 |
164,265 |
|
55 |
7.8 |
97,064 |
11,262 |
17,983 |
47,578 |
13,957 |
187,845 |
|
60 |
6.8 |
126,696 |
14,037 |
20,257 |
47,229 |
11,353 |
219,572 |
It is noted that the number of crashes decreases as the speed limit increases. This is likely due to the fact that the design of higher speed roads affords more generous dimensions and greater safety features than similar lower speed facilities.
The total crash costs are the sums of the vehicle-vehicle crash costs and the vehicle-pedestrian/cyclist crash costs.
|
Speed Limit (mph) |
Ped/Cyclist Crash Costs ($) |
Vehicle-Vehicle Crash Costs ($) |
Total Crash Costs ($) |
|---|---|---|---|
|
20 |
274,617 |
86,386 |
361,003 |
|
25 |
207,263 |
199,011 |
406,275 |
|
30 |
158,280 |
104,839 |
263,120 |
|
35 |
121,888 |
111,780 |
233,669 |
|
40 |
94,256 |
125,552 |
219,808 |
|
45 |
73,407 |
142,082 |
215,489 |
|
50 |
57,378 |
164,265 |
221,642 |
|
55 |
44,867 |
187,845 |
232,712 |
|
60 |
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 |
1,384,939 |
|
25 |
30.2 |
255,848 |
895,467 |
406,275 |
1,301,741 |
|
30 |
31.8 |
243,161 |
851,063 |
263,120 |
1,114,183 |
|
35 |
32.7 |
236,190 |
826,664 |
233,669 |
1,060,332 |
|
40 |
32.6 |
236,733 |
828,564 |
219,808 |
1,048,372 |
|
45 |
32.8 |
235,559 |
824,457 |
215,489 |
1,039,946 |
|
50 |
32.1 |
240,698 |
842,443 |
221,642 |
1,064,086 |
|
55 |
31.1 |
248,441 |
869,542 |
232,712 |
1,102,254 |
|
60 |
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 |
40 |
|
Illinois DOT |
40 |
|
Northwestern |
45 |
|
USLIMITS2 |
40 |
|
Optimal Speed |
45 |
|
Safe 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 |
24,900 |
|
15.20 |
25,500 |
|
20.87 |
25,000 |
|
21.35 |
29,500 |
|
Average |
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 |
57 |
|
15.00 |
63 |
57 |
57 |
60 |
60 |
|
18.10 |
65 |
60 |
60 |
64 |
83 |
|
19.70 |
59 |
56 |
56 |
60 |
59 |
|
21.00 |
54 |
51 |
51 |
55 |
8 |
|
Average |
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 |
11.49 |
|
|
12.17 |
11.96 |
|
|
12.38 |
12.78 |
|
|
12.76 |
13.69 |
|
|
13.80 |
13.78 |
|
|
13.96 |
14.06 |
|
|
14.07 |
14.89 |
|
|
14.10 |
15.56 |
|
|
14.34 |
16.19 |
|
|
14.36 |
16.28 |
|
|
14.89 |
16.45 |
|
|
15.46 |
17.43 |
|
|
15.67 |
17.51 |
|
|
15.89 |
18.16 |
|
|
16.13 |
18.27 |
|
|
16.96 |
18.37 |
|
|
17.00 |
18.54 |
|
|
17.89 |
18.92 |
|
|
18.05 |
|
|
|
18.14 |
|
|
|
18.21 |
|
|
|
20.25 |
|
|
Public Road |
14.35 (Iron Mountain Trail) |
13.56 (Scripps Poway Parkway) |
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 |
60 |
|
15.00 |
63 |
60 |
63 |
62 |
60 |
|
18.10 |
65 |
64 |
63 |
64 |
65 |
|
19.70 |
59 |
60 |
63 |
61 |
60 |
|
21.00 |
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 |
0 |
|
41 to 60 |
5 |
|
> 60 |
10 |
(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 |
55 |
|
15.00 |
60 |
54 |
55 |
|
18.10 |
65 |
59 |
60 |
|
19.70 |
60 |
54 |
1 55 |
|
21.00 |
55 |
50 |
50 |
The 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 |
56 |
|
15.00 |
55 |
57 |
|
18.10 |
60 |
60 |
|
19.70 |
55 |
56 |
|
21.00 |
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:
(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 |
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 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) |
|
-5 |
|
|
Commercial Access (Table 20) |
+5 |
|
|
|
Lane Width (Table 21) |
+5 |
|
|
|
Functional Classification (Table 22) |
|
-5 |
|
|
Median Type (Table 23) |
0 |
|
|
|
Shoulder Type and Width (Table 24) |
0 |
|
|
|
Pedestrian Activity (Table 25) |
|
-15 |
|
|
Parking Activity (Table 26) |
0 |
|
|
|
Roadway Alignment (Table 27) |
+ 10 |
|
|
|
Crash 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 + 0.0102SL + 0.00045X + 0.0000015ADT)
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 - Roadway Information Crash Data Information |
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 |
0.7794 |
|
45 |
0.0025 |
0.0044 |
0.0386 |
0.2188 |
0.7357 |
|
50 |
0.0037 |
0.0062 |
0.0495 |
0.2491 |
0.6915 |
|
55 |
0.0055 |
0.0088 |
0.0635 |
0.2816 |
0.6406 |
|
60 |
0.0080 |
0.0117 |
0.0788 |
0.3105 |
0.5910 |
|
65 |
0.0116 |
0.0160 |
0.1001 |
0.3390 |
0.5333 |
|
70 |
0.0200 |
0.1250 |
0.3651 |
0.4349 |
0.0550 |
|
75 |
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 |
3,366,388 |
|
A |
233,100 |
|
B |
46,620 |
|
C |
24,510 |
|
O |
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 |
254,810 |
|
45 |
19.1 |
160,524 |
19,563 |
34,324 |
102,288 |
36,344 |
353,044 |
|
50 |
20.1 |
250,006 |
29,008 |
46,319 |
122,547 |
35,948 |
483,829 |
|
55 |
21.1 |
391,076 |
43,327 |
62,529 |
145,784 |
35,045 |
677,760 |
|
60 |
22.2 |
598,601 |
60,619 |
81,655 |
169,156 |
34,023 |
944,054 |
|
65 |
23.4 |
913,386 |
87,236 |
109,154 |
194,346 |
32,308 |
1,336,429 |
|
70 |
24.6 |
1,657,202 |
717,189 |
418,953 |
262,370 |
3,506 |
3,059,219 |
|
75 |
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 |
12,326,291 |
|
45 |
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 |
11,783,329 |
|
60 |
29.0 |
3,398,493 |
11,894,724 |
944,054 |
12,838,778 |
|
65 |
26.5 |
3,720,556 |
13,021,947 |
1,336,429 |
14,358,376 |
|
70 |
24.1 |
4,090,876 |
14,318,066 |
3,059,219 |
17,377,286 |
|
75 |
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 |
55 |
|
Illinois DOT |
55 |
|
Northwestern |
55 |
|
USLIMITS2 |
55 |
|
Optimal Speed |
50 |
|
Safe System Speed |
50 |