Evaluating the safety effectiveness of downgrade warning signs on vehicle crashes on Wyoming mountain passes

Figures & data

Figure 1. Grade profile by MP for route ML36 (US 16)

Figure 2. Categories of warning signs (MUTCD)

Table 1. Summary statistic warning signs

Advance warning sign type	Frequency	Frequency/mile
Hill Sign W7-1	96	0.026
Hill Sign with grade value advisory sign (W7-1a)	43	0.012
Hill sign with lower gear advisory plaque (W7-1 + W7-2P)	619	0.168
Hill sign combination with downgrade distance advisory plaque (W7-1 + W7-3aP)	176	0.048
Hill sign combination with downgrade and distance advisory plaque (W7-1a + W7-3aP)	534	0.145
Truck Escape Ramp Sign (W7-4b)	544	0.147
Speed sign	377	0.102
Directional sign	1201	0.325
Directional and advisory speed plaque	2706	0.732
Chevron sign	362	0.098
Miscellaneous	1234	0.334
Warning signs within downgrade	305	1.773

Table 2. Summary statistics of major variables (categorical variables)

Category	Variable	Frequency	Percentage
Response	Truck crashes	233	3
	Non-truck crashes	6696	97
Presence of passing lane	Yes	201	16
	No	1030	84
Presence of warning sign	Yes	1027	83
	No	204	17
Vertical curve type	Sag	329	27
	Crest	337	27
	None	565	46

Table 3. Summary statistics of key variables (continuous variables)

Variable	Mean	Std. Dev.	Minimum	Maximum
Length (×1000 miles)	139.74	97.25	100	2460
ADTT	90.57	87.000	9.0	646.0
Average grade (%)	6.374	1.077	5.0	12.4
Superelevation (%)	0.0107	0.027	0.0	0.6
Deflection angle (radians)	26.31	32.393	0.0	205.2

Table 4. Estimated regression parameters for truck crash frequency

Parameter	Estimate	Standard Error	Wald chi-square	P-value
Intercept	−3.614	0.7431	21.78	<.0001
Average grade	0.277	0.0946	8.59	0.0027
Super elevation	0.047	0.0235	3.92	0.0385
Deflection angle	0.883	0.2080	18.02	<.0001
Passing lane	−0.641	0.3142	4.16	0.0408
ADTT	0.009	0.0012	54.51	<.0001
Warning signs within downgrade	0.452	0.1712	6.97	0.0084
Miscellaneous warning signs	0.705	0.2558	7.59	0.0059
Truck escape ramp sign	−0.572	0.2644	4.68	0.0305
Directional and speed plaque sign	−0.380	0.0613	38.41	0.0244
Hill sign combination with downgrade and distance advisory plaque (W7-1 + W7-3aP)	−0.452	0.1827	3.20	0.0190
Model fit statistics
Dispersion	3.654	0.66007	30.65	<.0001
AIC	1028.07
Log Likelihood	−389.480

Table 5. Estimated regression parameters for other vehicular (non-truck) crash frequency

Parameter	Category	Estimate	Standard Error	Wald chi-square	P-value
Intercept		1.538	0.1456	111.59	<.0001
Downgrade length		0.036	0.0053	46.29	<.0001
Curve type	Sag	−0.130	0.0852	2.33	0.1266
	Crest	−0.171	0.0863	3.92	0.0477
Super elevation		0.074	0.0174	17.86	<.0001
Horizontal tangent length		−0.012	0.0006	4.98	0.0257
Passing lane		−0.190	0.1010	3.55	0.0595
ADTT		0.002	0.0005	16.50	<.0001
Presence of downgrade warning sign		−0.3271	0.0965	11.49	0.0007
Hill Sign (W7-1)		−0.506	0.1381	13.43	0.0002
Directional and speed advisory sign		−0.0842	0.0170	24.5	<.0001
Chevron warning sign		−0.1129	0.0578	3.82	0.0501
Model Fit Statistics
Dispersion		1.2593	0.062	440.51	<.0001
AIC		6540
Log Likelihood		8621

Figure 3. Impact of independent variables on truck crashes

Figure 4. Impact of independent variables on non-truck crashes

Table 6. Elasticity of crash prediction model for truck and other vehicle crashes truck frequency model elasticities non-truck frequency model elasticities

Parameter	Elasticity or Pseudo Elasticity (%)	Parameter	Elasticity or Pseudo Elasticity (%)
Average grade	171.40	Curve length	19.90
Super elevation	1.94	Curve type (crest)	−15.72
Deflection angle	22.50	Super elevation	2.84
Passing lane	−47.32	Passing lane	−17.30
ADTT	155.70	Horizontal tangent length	8.63
Warning signs within downgrade	7.47	ADTT	34.60
Miscellaneous warning signs	29.37	Presence of downgrade warning sign	−27.89
Truck escape ramp sign	−5.69	Hill Sign (W7-1)	−9.11
Directional and speed advisory sign	−16.80	Directional and speed advisory sign	−9.50
Hill sign combination with downgrade and distance advisory plaque (W7-1 + W7-3aP)	−11.10	Chevron warning sign	−2.42

Figure 5. Bar chart for elasticities (truck crashes)

Figure 6. Bar chart for elasticities (non-truck crashes)