Highway Design and Safety Consequences: A Case Study of Interstate Highway Vertical Grades

Table 2

(a) Model estimation results for Rural Interstates

Variables	Fatal		Injury		PDO
	coefficient	t value	coefficient	t value	coefficient	t value
Constant	–12.8386	–4.30	–22.7048	–14.24	–19.8081	–14.11
Segment length (miles)	0.2146	3.98	0.1548	6.24	0.1540	7.05
Natural log of annual average daily traffic	1.4462	5.13	0.8843	6.48	0.8234	6.85
Average vertical grade (%)			0.0963	3.27	0.0636	2.46
Left shoulder width (ft.)	–1.1704	–10.81	–0.6364	–18.50	–0.7372	–24.38
Median width (ft.)			–0.0123	–2.91	–0.0131	–3.52
Number of lanes			3.4596	15.66	3.4045	17.53
	Number of observations = 317;

(b) Model estimation results for Urban Interstates

Variables	Fatal		Injury		PDO
	coefficient	t value	coefficient	t value	coefficient	t value
Constant	–5.1638	–8.25	–28.7188	–6.34	–24.2710	–5.77
Segment length (miles)	0.2327	9.16
Natural logarithm of annual average daily traffic			2.8430	8.19	2.2157	8.28
Lane width (ft.)			–0.9093	–3.65	–0.6286	–3.06
Right shoulder width (ft.)			1.2243	5.01	1.1272	6.00
IRI (in/mile)	0.02317	3.25
Average vertical grade (%)	0.5034	5.12	0.2086	1.96	0.1852	1.86
Left shoulder width (ft.)	–0.2087	–5.01	–0.08187	–1.99	–0.09313	–2.38
Number of lanes	–0.2480	–2.45
Average vertical curve grade Lane width (Interaction term)			0.02367	3.19	0.02615	3.72
	Number of observations = 101;