Reliability and Overall Standard Deviation

Rarely does the actual traffic loading to failure equal the predicted traffic loading; the difference is due to the deviations that exist. These deviations include (1) lack of fit of the AASHTO design equations, since these are empirical equations; (2) variations in construction, which cause variations in the equation input factors such as the strength and thickness of pavement layers; and (3) variations in the predicted traffic (see App. EE of Vol. 2 of the AASHTO Guide for Design of Pavement Structures, August 1986 edition). The AASHTO equations account for these variations by multiplying the predicted traffic by a safety factor. The safety factor is determined by the reliability desired and

TABLE 3.5 Axle Load Equivalency Factors for Flexible Pavements, Tandem Axles, and pt of 2.5

Axle load		Pavement structural number (SN)
kips	kN	1	2	3	4	5	6
2	9	0.0001	0.0001	0.0001	0.0000	0.0000	0.0000
4	18	0.0005	0.0005	0.0004	0.0003	0.0003	0.0002
6	27	0.002	0.002	0.002	0.001	0.001	0.001
8	36	0.004	0.006	0.005	0.004	0.003	0.003
10	44	0.008	0.013	0.011	0.009	0.007	0.006
12	53	0.015	0.024	0.023	0.018	0.014	0.013
14	62	0.026	0.041	0.042	0.033	0.027	0.024
16	71	0.044	0.065	0.070	0.057	0.047	0.043
18	80	0.070	0.097	0.109	0.092	0.077	0.070
20	89	0.107	0.141	0.162	0.141	0.121	0.110
22	98	0.160	0.198	0.229	0.207	0.180	0.166
24	107	0.231	0.273	0.315	0.292	0.260	0.242
26	116	0.327	0.370	0.420	0.401	0.364	0.342
28	125	0.451	0.493	0.548	0.534	0.495	0.470
30	133	0.611	0.648	0.703	0.695	0.658	0.633
32	142	0.813	0.843	0.889	0.887	0.857	0.834
34	151	1.06	1.08	1.11	1.11	1.09	1.08
36	160	1.38	1.38	1.38	1.38	1.38	1.38
38	169	1.75	1.73	1.69	1.68	1.70	1.73
40	178	2.21	2.16	2.06	2.03	2.08	2.14
42	187	2.76	2.67	2.49	2.43	2.51	2.61
44	196	3.41	3.27	2.99	2.88	3.00	3.16
46	205	4.18	3.98	3.58	3.40	3.55	3.79
48	214	5.08	4.80	4.25	3.98	4.17	4.49
50	222	6.12	5.76	5.03	4.64	4.86	5.28
52	231	7.33	6.87	5.93	5.38	5.63	6.17
54	240	8.72	8.14	6.95	6.22	6.47	7.15
56	249	10.3	9.6	8.1	7.2	7.4	8.2
58	258	12.1	11.3	9.4	8.2	8.4	9.4
60	267	14.2	13.1	10.9	9.4	9.6	10.7
62	276	16.5	15.3	12.6	10.7	10.8	12.1
64	285	19.1	17.6	14.5	12.2	12.2	13.7
66	294	22.1	20.3	16.6	13.8	13.7	15.4
68	302	25.3	23.3	18.9	15.6	15.4	17.2
70	311	29.0	26.6	21.5	17.6	17.2	19.2
72	320	33.0	30.3	24.4	19.8	19.2	21.3
74	329	37.5	34.4	27.6	22.2	21.3	23.6
76	338	42.5	38.9	31.1	24.8	23.7	26.1
78	347	48.0	43.9	35.0	27.8	26.2	28.8
80	356	54.0	49.4	39.2	30.9	29.0	31.7
82	365	60.6	55.4	43.9	34.4	32.0	34.8
84	374	67.8	61.9	49.0	38.2	35.3	38.1
86	383	75.7	69.1	54.5	42.3	38.8	41.7
88	391	84.3	76.9	60.6	46.8	42.6	45.6
90	400	93.7	85.4	67.1	51.7	46.8	49.7

Axle load		Pavement structural number (SN)
kips	kN	1	2	3	4	5	6
2	9	0.0000	0.0000	0.0000	0.0000	0.0000	0.0000
4	18	0.0002	0.0002	0.0002	0.0001	0.0001	0.0001
6	27	0.0006	0.0007	0.0005	0.0004	0.0003	0.0003
8	36	0.001	0.002	0.001	0.001	0.001	0.001
10	44	0.003	0.004	0.003	0.002	0.002	0.002
12	53	0.005	0.007	0.006	0.004	0.003	0.003
14	62	0.008	0.012	0.010	0.008	0.006	0.006
16	71	0.012	0.019	0.018	0.013	0.011	0.010
18	80	0.018	0.029	0.028	0.021	0.017	0.016
20	89	0.027	0.042	0.042	0.032	0.027	0.024
22	98	0.038	0.058	0.060	0.048	0.040	0.036
24	107	0.053	0.078	0.084	0.068	0.057	0.051
26	116	0.072	0.103	0.114	0.095	0.080	0.072
28	125	0.098	0.133	0.151	0.128	0.109	0.099
30	133	0.129	0.169	0.195	0.170	0.145	0.133
32	142	0.169	0.213	0.247	0.220	0.191	0.175
34	151	0.219	0.266	0.308	0.281	0.246	0.228
36	160	0.279	0.329	0.379	0.352	0.313	0.292
38	169	0.352	0.403	0.461	0.436	0.393	0.368
40	178	0.439	0.491	0.554	0.533	0.487	0.459
42	187	0.543	0.594	0.661	0.644	0.597	0.567
44	196	0.666	0.714	0.781	0.769	0.723	0.692
46	205	0.811	0.854	0.918	0.911	0.868	0.838
48	214	0.979	1.015	1.072	1.069	1.033	1.005
50	222	1.17	1.20	1.24	1.25	1.22	1.20
52	231	1.40	1.41	1.44	1.44	1.43	1.41
54	240	1.66	1.66	1.66	1.66	1.66	1.66
56	249	1.95	1.93	1.90	1.90	1.91	1.93
58	258	2.29	2.25	2.17	2.16	2.20	2.24
60	267	2.67	2.60	2.48	2.44	2.51	2.58
62	276	3.09	3.00	2.82	2.76	2.85	2.95
64	285	3.57	3.44	3.19	3.10	3.22	3.36
66	294	4.11	3.94	3.61	3.47	3.62	3.81
68	302	4.71	4.49	4.06	3.88	4.05	4.30
70	311	5.38	5.11	4.57	4.32	4.52	4.84
72	320	6.12	5.79	5.13	4.80	5.03	5.41
74	329	6.93	6.54	5.74	5.32	5.57	6.04
76	338	7.84	7.37	6.41	5.88	6.15	6.71
78	347	8.83	8.28	7.14	6.49	6.78	7.43
80	356	9.92	9.28	7.95	7.15	7.45	8.21
82	365	11.1	10.4	8.8	7.9	8.2	9.0
84	374	12.4	11.6	9.8	8.6	8.9	9.9
86	383	13.8	12.9	10.8	9.5	9.8	10.9
88	391	15.4	14.3	11.9	10.4	10.6	11.9
90	400	17.1	15.8	13.2	11.3	11.6	12.9

TABLE 3.7 Axle Load Equivalency Factors for Flexible Pavements, Single Axles, and pt of 3.0

Axle load		Pavement structural number (SN)
kips	kN	1	2	3	4	5	6
2	9	0.0008	0.0009	0.0006	0.0003	0.0002	0.0002
4	18	0.004	0.008	0.006	0.004	0.002	0.002
6	27	0.014	0.030	0.028	0.018	0.012	0.010
8	36	0.035	0.070	0.080	0.055	0.040	0.034
10	44	0.082	0.132	0.168	0.132	0.101	0.086
12	53	0.173	0.231	0.296	0.260	0.212	0.187
14	62	0.332	0.388	0.468	0.447	0.391	0.358
16	71	0.594	0.633	0.695	0.693	0.651	0.622
18	80	1.00	1.00	1.00	1.00	1.00	1.00
20	89	1.60	1.53	1.41	1.38	1.44	1.51
22	98	2.47	2.29	1.96	1.83	1.97	2.16
24	107	3.67	3.33	2.69	2.39	2.60	2.96
26	116	5.29	4.72	3.65	3.08	3.33	3.91
28	125	7.43	6.56	4.88	3.93	4.17	5.00
30	133	10.2	8.9	6.5	5.0	5.1	6.3
32	142	13.8	12.0	8.4	6.2	6.3	7.7
34	151	18.2	15.7	10.9	7.8	7.6	9.3
36	160	23.8	20.4	14.0	9.7	9.1	11.0
38	169	30.6	26.2	17.7	11.9	11.0	13.0
40	178	38.8	33.2	22.2	14.6	13.1	15.3
42	187	48.8	41.6	27.6	17.8	15.5	17.8
44	196	60.6	51.6	34.0	21.6	18.4	20.6
46	205	74.7	63.4	41.5	26.1	21.6	23.8
48	214	91.2	77.3	50.3	31.3	25.4	27.4
50	222	110.	94.	61.	37.	30.	32.

Source: Guide for Design of Pavement Structures, American Association of State Highway

and Transportation Officials, Washington, D. C., 1993, with permission.

the amount of total variation or the overall standard deviation. AASHTO recommends the following reliability based on the functional classification of the road:

	Recommended
	level of reliability
Functional classification	Urban	Rural
Interstate/freeway	85-99.9	80-99.9
Principal arterials	80-99	75-95
Collectors	80-95	75-95
Local	50-80	50-80

Overall standard deviation values recommended by AASHTO are 0.30 to 0.40 for rigid pavements and 0.40 to 0.50 for flexible pavements. The lower values are more appropriate when traffic predictions are more reliable. Values derived from the AASHTO Road Test are 0.39 for rigid pavements and 0.49 for flexible pavements.

Axle load		Pavement structural number (SN)
kips	kN	1	2	3	4	5	6
2	9	0.0002	0.0002	0.0001	0.0001	0.0000	0.0000
4	18	0.001	0.001	0.001	0.000	0.000	0.000
6	27	0.003	0.004	0.003	0.002	0.001	0.001
8	36	0.006	0.011	0.009	0.005	0.003	0.003
10	44	0.011	0.024	0.020	0.012	0.008	0.007
12	53	0.019	0.042	0.039	0.024	0.017	0.014
14	62	0.031	0.066	0.068	0.045	0.032	0.026
16	71	0.049	0.096	0.109	0.076	0.055	0.046
18	80	0.075	0.134	0.164	0.121	0.090	0.076
20	89	0.113	0.181	0.232	0.182	0.139	0.119
22	98	0.166	0.241	0.313	0.260	0.205	0.178
24	107	0.238	0.317	0.407	0.358	0.292	0.257
26	116	0.333	0.413	0.517	0.476	0.402	0.360
28	125	0.457	0.534	0.643	0.614	0.538	0.492
30	133	0.616	0.684	0.788	0.773	0.702	0.656
32	142	0.817	0.870	0.956	0.953	0.896	0.855
34	151	1.07	1.10	1.15	1.15	1.12	1.09
36	160	1.38	1.38	1.38	1.38	1.38	1.38
38	169	1.75	1.71	1.64	1.62	1.66	1.70
40	178	2.21	2.11	1.94	1.89	1.98	2.08
42	187	2.75	2.59	2.29	2.19	2.33	2.50
44	196	3.39	3.15	2.70	2.52	2.71	2.97
46	205	4.15	3.81	3.16	2.89	3.13	3.50
48	214	5.04	4.58	3.70	3.29	3.57	4.07
50	222	6.08	5.47	4.31	3.74	4.05	4.70
52	231	7.27	6.49	5.01	4.24	4.57	5.37
54	240	8.65	7.67	5.81	4.79	5.13	6.10
56	249	10.2	9.0	6.7	5.4	5.7	6.9
58	258	12.0	10.6	7.7	6.1	6.4	7.7
60	267	14.1	12.3	8.9	6.8	7.1	8.6
62	276	16.3	14.2	10.2	7.7	7.8	9.5
64	285	18.9	16.4	11.6	8.6	8.6	10.5
66	294	21.8	18.9	13.2	9.6	9.5	11.6
68	302	25.1	21.7	15.0	10.7	10.5	12.7
70	311	28.7	24.7	17.0	12.0	11.5	13.9
72	320	32.7	28.1	19.2	13.3	12.6	15.2
74	329	37.2	31.9	21.6	14.8	13.8	16.5
76	338	42.1	36.0	24.3	16.4	15.1	17.9
78	347	47.5	40.6	27.3	18.2	16.5	19.4
80	356	53.4	45.7	30.5	20.1	18.0	21.0
82	365	60.0	51.2	34.0	22.2	19.6	22.7
84	374	67.1	57.2	37.9	24.6	21.3	24.5
86	383	74.9	63.8	42.1	27.1	23.2	26.4
88	391	83.4	71.0	46.7	29.8	25.2	28.4
90	400	92.7	78.8	51.7	32.7	27.4	30.5

TABLE 3.9 Axle Load Equivalency Factors for Flexible Pavements, Triple Axles, and pt of 3.0

Axle load		Pavement structural number (SN)
kips	kN	1	2	3	4	5	6
2	9	0.0001	0.0001	0.0001	0.0000	0.0000	0.0000
4	18	0.0005	0.0004	0.0003	0.0002	0.0001	0.0001
6	27	0.001	0.001	0.001	0.001	0.000	0.000
8	36	0.003	0.004	0.002	0.001	0.001	0.001
10	44	0.005	0.008	0.005	0.003	0.002	0.002
12	53	0.007	0.014	0.010	0.006	0.004	0.003
14	62	0.011	0.023	0.018	0.011	0.007	0.006
16	71	0.016	0.035	0.030	0.018	0.013	0.010
18	80	0.022	0.050	0.047	0.029	0.020	0.017
20	89	0.031	0.069	0.069	0.044	0.031	0.026
22	98	0.043	0.090	0.097	0.065	0.046	0.039
24	107	0.059	0.116	0.132	0.092	0.066	0.056
26	116	0.079	0.145	0.174	0.126	0.092	0.078
28	125	0.104	0.179	0.223	0.168	0.126	0.107
30	133	0.136	0.218	0.279	0.219	0.167	0.143
32	142	0.176	0.265	0.342	0.279	0.218	0.188
34	151	0.226	0.319	0.413	0.350	0.279	0.243
36	160	0.286	0.382	0.491	0.432	0.352	0.310
38	169	0.359	0.456	0.577	0.524	0.437	0.389
40	178	0.447	0.543	0.671	0.626	0.536	0.483
42	187	0.550	0.643	0.775	0.740	0.649	0.593
44	196	0.673	0.760	0.889	0.865	0.777	0.720
46	205	0.817	0.894	1.014	1.001	.920	.865
48	214	0.984	1.048	1.152	1.148	1.080	1.030
50	222	1.18	1.23	1.30	1.31	1.26	1.11
52	231	1.40	1.43	1.47	1.48	1.45	1.43
54	240	1.66	1.66	1.66	1.66	1.66	1.66
56	249	1.95	1.92	1.86	1.85	1.88	1.91
58	258	2.28	2.21	2.09	2.06	2.13	2.20
60	267	2.66	2.54	2.34	2.28	2.39	2.50
62	276	3.08	2.92	2.61	2.52	2.66	2.84
64	285	3.56	3.33	2.92	2.77	2.96	3.19
66	294	4.09	3.79	3.25	3.04	3.27	3.58
68	302	4.68	4.31	3.62	3.33	3.60	4.00
70	311	5.34	4.88	4.02	3.64	3.94	4.44
72	320	6.08	5.51	4.46	3.97	4.31	4.91
74	329	6.89	6.21	4.94	4.32	4.69	5.40
76	338	7.78	6.98	5.47	4.70	5.09	5.93
78	347	8.76	7.83	6.04	5.11	5.51	6.48
80	356	9.84	8.75	6.67	5.54	5.96	7.06
82	365	11.0	9.8	7.4	6.0	6.4	7.7
84	374	12.3	10.9	8.1	6.5	6.9	8.3
86	383	13.7	12.1	8.9	7.0	7.4	9.0
88	391	15.3	13.4	9.8	7.6	8.0	9.6
90	400	16.9	14.8	10.7	8.2	8.5	10.4

Axle load				Slab thickness D, in
kips	kN	6	7	8	9	10	11	12	13	14
2	9	0.0002	0.0002	0.0002	0.0002	0.0002	0.0002	0.0002	0.0002	0.0002
4	18	0.002	0.002	0.002	0.002	0.002	0.002	0.002	0.002	0.002
6	27	0.011	0.010	0.010	0.010	0.010	0.010	0.010	0.010	0.010
8	36	0.035	0.033	0.032	0.032	0.032	0.032	0.032	0.032	0.032
10	44	0.087	0.084	0.082	0.081	0.080	0.080	0.080	0.080	0.080
12	53	0.186	0.180	0.176	0.175	0.174	0.174	0.173	0.173	0.173
14	62	0.353	0.346	0.341	0.338	0.337	0.336	0.336	0.336	0.336
16	71	0.614	0.609	0.604	0.601	0.599	0.599	0.598	0.598	0.598
18	80	1.00	1.00	1.00	1.00	1.00	1.00	1.00	1.00	1.00
20	89	1.55	1.56	1.57	1.58	1.58	1.59	1.59	1.59	1.59
22	98	2.32	2.32	2.35	2.38	2.40	2.41	2.41	2.41	2.42
24	107	3.37	3.34	3.40	3.47	3.51	3.53	3.54	3.55	3.55
26	116	4.76	4.69	4.77	4.88	4.97	5.02	5.04	5.06	5.06
28	125	6.58	6.44	6.52	6.70	6.85	6.94	7.00	7.02	7.04
30	133	8.92	8.68	8.74	8.98	9.23	9.39	9.48	9.54	9.56
32	142	11.9	11.5	11.5	11.8	12.2	12.4	12.6	12.7	12.7
34	151	15.5	15.0	14.9	15.3	15.8	16.2	16.4	16.6	16.7
36	160	20.1	19.3	19.2	19.5	20.1	20.7	21.1	21.4	21.5
38	169	25.6	24.5	24.3	24.6	25.4	26.1	26.7	27.1	27.4
40	178	32.2	30.8	30.4	30.7	31.6	32.6	33.4	34.0	34.4
42	187	40.1	38.4	37.7	38.0	38.9	40.1	41.3	42.1	42.7
44	196	49.4	47.3	46.4	46.6	47.6	49.0	50.4	51.6	52.4
46	205	60.4	57.7	56.6	56.7	57.7	59.3	61.1	62.6	63.7
48	214	73.2	69.9	68.4	68.4	69.4	71.2	73.3	75.3	76.8
50	222	88.0	84.1	82.2	82.0	83.0	84.9	87.4	89.8	91.7

Conversion: 1 in = 25.4 mm.

Source: Guide for Design of Pavement Structures, American Association of State Highway and

Transportation Officials, Washington, D. C., 1993, with permission.

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Reliability and Overall Standard Deviation

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