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Appendix D. Safety Analyses on Multilane Highways

Publication Number: FHWA-HRT-12-048 Date: November 2013

Publication Number: FHWA-HRT-12-048
Date: November 2013

Pavement Marking Demonstration Projects: State of Alaska and State of Tennessee

APPENDIX D. SAFETY ANALYSES ON MULTILANE HIGHWAYS

The Illinois freeway crash data were compiled from 2001-2006 from 571 segments (708.1 mi). Table 93 provides a summary of crash rates for those 571 freeway segments. Crashes coded as "intersection" were removed from all crash counts.

Table 93. Summary of Illinois 2001-2006 freeway crash data.

Variable	4-Inch Edge Lines 4-Inch Skip Lines	4-Inch Edge Lines 6-Inch Skip Lines	5-Inch Edge Lines 5-Inch Skip Lines
Total segment length	593.1	21.4	93.6
Number of years	6	6	6
Total number of accidents	31,189	432	1,751
Crash Type	Crash Rates (per million vehicle miles)
Total	0.84	0.50	0.53
F+I	0.17	0.12	0.10
PDO	0.67	0.39	0.43
Daytime	0.47	0.21	0.23
Nighttime	0.33	0.26	0.26
Daytime F+I	0.10	0.07	0.06
Nighttime F+I	0.06	0.04	0.04
Wet	0.10	0.04	0.05
Wet nighttime	0.04	0.02	0.03
Single-vehicle	0.38	0.39	0.42
Single-vehicle wet	0.06	0.04	0.04
Single-vehicle nighttime	0.20	0.22	0.24
Older driver (≥ 55 years old)	0.16	0.09	0.11
Opposite direction	0.01	0.00	0.00
Sideswipe same direction	0.14	0.04	0.05
Fixed object	0.20	0.13	0.10

The Michigan freeway crash data were compiled from 2001-2007 from 508 segments (1,067.4 mi). The annual aggregated crash counts from those 508 freeway segments are provided in table 94. Crashes coded as "intersection" or "interchange" have been removed from all crash counts.

Table 94. Annual aggregated crash counts over 508 freeway segments (1,067.4 mi)
in Michigan for 2001-2007.

Crash Type	Year
Crash Type	2001	2002	2003	2004	2005	2006	2007
Total	1,364	1,347	1,466	1,692	1,556	1,529	1,489
F+I	294	288	288	313	301	291	272
PDO	1,070	1,059	1,178	1,379	1,255	1,238	1,217
Daytime	856	854	879	1,058	963	900	881
Nighttime	400	387	448	519	480	498	484
Daytime F+I	207	188	187	219	217	200	198
Nighttime F+I	68	82	82	77	65	72	60
Wet	202	200	176	217	137	245	163
Wet nighttime	56	44	53	73	37	64	48
Wet F+I	50	38	29	49	30	62	34
Single-vehicle	774	696	815	993	897	948	977
Single-vehicle wet	112	98	113	141	83	158	113
Single-vehicle nighttime	327	280	357	423	399	416	424
Single-vehicle F+I	163	133	149	162	164	146	151
Single-vehicle nighttime F+I	53	51	53	51	41	48	40
Single-vehicle wet F+I	31	17	17	31	17	38	22

Note: Crashes in 2004 are not included in the safety analysis because 2004 is the year of wider line installation.

The research team performed a safety evaluation of Michigan freeway crash data using an interrupted time series approach that introduces time as a variable to control for baseline trend and intervention (installation of wider lines) as a variable to estimate the effect of the wider lines. Because the time series plots of freeway crashes (aggregated crashes over all segments) by year indicated a possible change in the trend (not just in the level) in crashes after the intervention, a new variable, "time after intervention," coded as "0" before the intervention and (time - t₀), where t₀ is the year of the intervention, was also included to estimate the change in the trend in the expected number of crashes. In addition to time, intervention, and time after intervention, lane width, terrain, log(AADT), log(segment length), and log(number of rainy days) were included as predictors in the negative binomial regression model. GEEs were used as an estimation method to account for correlations in crash counts from multiple years over segments. Table 95 contains the results of applying interrupted time series approaches to the Michigan non-intersection/interchange non-winter month crash data from 508 segments (1,067.4 mi) of freeways with 3 years (2001-2003) of before and 3 years (2005-2007) of after data. It can be observed from the table that there was no statistically significant change in the level of or trend in the expected number of freeway crashes after the installation of wider lines (p-values for intervention or time after intervention were all greater than 0.05), regardless of the crash types considered in the table. That is, no consistent or statistically significant safety effects of wider lines were observed for the Michigan freeway crash data.

Table 95. Results of interrupted time series approaches applied to the Michigan non-intersection/interchange
non-winter month crash data.

Variable	Intercept	Time	Intervention	Time After Intervention	Lane Width	Terrain	Log (AADT)	Log (Segment Length)	Log (Number of Rainy Days)
Total crashes	-6.7012	-0.0089	0.0831	0.0208	0.0064	-0.0806	0.5997	1.3521	0.1682
F+I	-15.9347	-0.0715	0.1936	0.0524	0.3270	0.2609	0.9636	1.4891	0.0831
PDO	-6.0226	0.0117	0.0640	-0.0002	-0.0593	-0.0728	0.5784	1.3357	0.1973
Daytime	-14.4469	-0.0252	0.1077	0.0339	0.3934	-0.0555	0.8498	1.3990	0.1700
Nighttime	-0.0588	0.0121	0.1723	-0.0182	-0.3580	-0.0798	0.3279	1.3128	0.0140
Daytime F+I	-15.8048	-0.1267	0.3517	0.1211	0.1888	0.4336	1.0686	1.5658	0.0220
Nighttime F+I	-19.2711	0.0725	-0.2293	-0.1077	0.5150	0.3201	0.9362	1.4485	0.1507
Wet	-15.8378	-0.0059	-0.1984	0.1047	0.0310	-0.3612	0.7493	1.2577	1.3567
Wet nighttime	-14.7010	0.0840	-0.3696	0.0035	0.0785	-0.5859	0.4336	1.0224	1.5177
Wet F+I	-26.6570	-0.2133	0.4955	0.2534	0.4938	0.5057	0.9269	1.2374	1.4791
Single-vehicle	-0.8200	-0.0041	0.0971	0.0567	-0.1900	-0.1669	0.2324	1.3268	0.0815
Single-vehicle wet	-8.3744	0.0735	-0.3716	0.0903	-0.2281	-0.4371	0.4063	1.1960	1.1056
Single-vehicle nighttime	0.7417	0.0205	0.1645	-0.0008	-0.3031	-0.1165	0.1442	1.2660	0.0452
Single-vehicle V+I	-10.7619	-0.0801	0.2421	0.0511	0.2354	0.2252	0.5641	1.4455	-0.0618
Single-vehicle nighttime F+I	-16.4857	0.0040	-0.1630	-0.0186	0.5384	0.0848	0.5376	1.3888	0.2870
Single-vehicle wet F+I	-19.3663	-0.2613	0.5892	0.3502	0.2290	0.6558	0.6542	1.1711	1.0341

Note: GEE approach was used as an estimation method. Bold represents statistically significant results at α = 0.05.

Page Owner: Office of Research, Development, and Technology, Office of Infrastructure, RDT

Topics: research, infrastructure, pavements and materials
Keywords: research, infrastructure, pavements and materials, Acrylic waterborne paint, Durability, Environmental impacts, Pavement markings, Retroreflectivity, State bidding procedures, Wider edge lines
TRT Terms: research, facilities, transportation, highway facilities, roads, parts of roads, pavements
Scheduled Update: Archive - No Update needed

This page last modified on 08/01/2014