Roadway and environment-related variables

Some roadway and environmental characteristics were found significant in the exploratory models. In that analysis, the roadway characteristics had more relative significance than the environment-related variables. The road surface condition was found significant, but not the weather variable. This suggests that the severe injury outcome is affected to a degree by roadway characteristics that could be improved by engineers in the design, construction and maintenance of multilane high-speed arterial corridors. On the other hand, weather related variables are indirectly related to the road surface conditions, the friction course and skid resistance. This interaction is appreciated in the wet pavement crash analysis discussed ahead.

Table 5-18 Road speed limit by injury severity for vehicle-driver sections 1 and 2

Driver-vehicle section 1 Driver-vehicle section 2 Speed limit1 Severe driver1

Frequency

Percent 93.59% 6.41% 120421 100.00

% Test of independence p-value=<.0001, Contingency Coefficien =0.1118 t

Speed limit2 Severe driver2 Frequency

Percent 94.34% 5.66% 127819 100.00%

Test of independence p-value=<.0001, Contingency Coefficien =0.0685 t

Figure 5-1 – Video Log snapshot of an arterial corridor with 65 mph speed limit: State Road 10 in Gadsen county (Rdwyid 50030000 Direction: East MP: 6.57)

The speed limit of a road is one of the most important design parameters and controls other aspects not seen in the crash information analyzed in this investigation. This analysis is limited to multilane high-speed arterials. However, the crashes at intersections involve other roads with speed limits different from those of the arterials. The multilane arterial corridors with speed limits of 40 and 45 mph carry a major proportion (65.56%) of the severe injuries, as shown in Table 5-18 above. However, the proportions of severe injuries for roads with higher speeds range from 7.87% to 18.12%, which suggest an increased severe injury risk for crash involvements in higher speed (50-70 mph) roads. On the other hand the proportion of severe

(9.38% and 9.51%). This shows an increased risk for the minor roads, but not as high as the one for the higher speed roads (especially 60-70 mph).

Another area of interest was the injury severity outcome for drivers entering the arterial corridor from other intersecting roads. Table 5-18 above shows a considerable increase in the proportion of severe injuries for drivers on roads with higher speed limits (60-70 mph). These might be intersecting freeways (intersection related crash) or 60 mph multilane arterials found in rural areas, such as a segment in State Road 10 in Gadsen county, shown in Figure 5-1 above and and State Road 25 in Alachua county previously shown in Figure 3-1. Also, lower speed intersecting roads (usually minor roads) have a considerable proportion of severe driver injuries.

A possible interaction between contributing cause and the speed limits was investigated in the final analysis to determine whether engineering or educational countermeasures would be more effective in these two cases, which amount to 14% of the total severe injuries in this analysis.

Table 5-19 Road lighting condition by injury severity for vehicle-driver sections 1 and 2

Driver-vehicle section 1 Driver-vehicle section 2 Lighting

lighting (93.48) (6.52) 26200 21.76%

5439 882 Dark without

street

lighting (86.05) (13.95) 6321 5.25%

Total 112700 7721

Percent 93.59% 6.41% 120421 100.00%

Test of independence p-value=<.0001,

lighting (94.80) (5.20) 27842 21.78%

5500 560 Dark without

street

lighting (90.76) (9.24) 6060 4.74%

Total 120589 7230

Percent 94.34% 5.66% 127819 100.00%

Test of independence p-value=<.0001,

Changes in lighting conditions on multilane high-speed arterial corridors were a contributing factor with moderate to minor relevance in the exploratory analysis. Road lighting maintenance and its importance in preventing severe crashes at night has been a recent topic of discussion for the FDOT. Multilane high speed roads under state jurisdictions are more likely to have better lighting conditions, especially in urban areas. Better lighting conditions keep the drivers severe injuries in proportions of 6.52% (driver 1) and 5.20% (driver 2) comparable to daylight crashes for both driver sections, as shown in Table 5-19 above. The prejudicial effects of lack of street lighting are evident. The proportion of severe crashes at night when there is no lighting (13.95% for driver 1 and 9.24% for driver 2) is almost double the daylight rates. The benefits of improvements in street lighting could potentially reduce up to 238 severe crashes each year, when holding all other conditions constant. This figure is just an incomplete estimate because it does not include the driver sections other than the first two, but it is nonetheless significant when considering the total costs of severe crashes. Additional data from RCI will better describe these benefits in the final analysis.

Table 5-20 Rural and urban land use by injury severity for vehicle-driver sections 1 and 2

Driver-vehicle section 1 Driver-vehicle section 2 Rural/Urban Severe_driver1

Percent 93.59% 6.41% 120421 100.00%

Test of independence p-value=<.0001,

Percent 94.34% 5.66% 127819 100.00%

Test of independence p-value=<.0001, Contingency Coefficient=-0.0381

The literature agrees on the importance of the land use in certain road and traffic conditions that affect crash occurrence and severity. In the exploratory models it was a significant contributing factor. Many design characteristics and traffic conditions depend on the land use and Table 5-20 above shows that crashes occurring in rural areas account for 56.40% of the severe crashes in this analysis. Meanwhile more crash involvements were reported in urban areas, which account for 53% of the driver involvements. Of the crash involvements occurring on roads in rural areas, 7.99% (section 1) and 6.60% (section 2) result in severe injuries. The relative severe involvement ratio between the drivers involved in rural and urban crashes is 1.48 for all the involvements under study. Excluding other factors, there is 1.48 times the chance of a severe injury per driver involvement on roads in rural areas versus each involvement on roads in urban areas. It could be inferred that a set of conditions in the rural areas contribute to a significantly higher rates of severe involvements. Other design characteristics will complement these results and point to effective countermeasures tailored to different land uses.

Table 5-21 Type of shoulder by injury severity for vehicle-driver sections 1 and 2

Driver-vehicle section 1 Driver-vehicle section 2 Type of

Shoulder Severe driver1 Frequency

Percent 93.59% 6.41% 120421 100.00%

Test of independence p-value=<.0001, Contingency Coefficien =0.0427 t

Type of

Shoulder Severe driver2 Frequency

Percent 94.34% 5.66% 127819 100.00%

Test of independence p-value=<.0001, Contingency Coefficien =0.0189 t

Another road characteristic found significant in the exploratory models was the type of shoulder. Outside shoulders have been incorporated into road design mainly as a safety feature, but it has also proven to facilitate traffic flow as a rest area for incident management. As shown in Table 5-21 above, crashes on roads with curb shoulders report lower proportions of severe injuries, while roads with unpaved shoulders register the highest percentages (between 6.4 and 8.1 %). The lack of paved shoulder is suspected to be a contributor to roadside single vehicle crashes, which tend to cause severe injury. Curb shoulders close to the edge of the traveled way are dangerous at high speeds. However, the presence of curb and gutter also indicate urban designs, which usually are better illuminated, carry more traffic and have lower operating speeds.

These unobserved factors might be part of the perceived benefit of curbed shoulders. The shoulder width data may clarify the relationship between shoulders and driver injury severity.

Table 5-22 Road Surface Conditions by injury severity for vehicle-driver sections 1 and 2

Driver-vehicle section 1 Driver-vehicle section 2 Road Surface

Condition Severe driver1 Frequency

Percent 93.59% 6.41% 120421 100.00

% Test of independence p-value=<.0001,

Road Surface

Condition Severe driver2 Frequency

Percent 94.34% 5.66% 127819 100.00

% Test of independence p-value=0.0002,

The road surface condition reflects the prevailing weather conditions. Although the relative importance of this variable is lower than the rest, it might prove to be an important interaction term. Also, around the state of Florida severe weather events such as rainstorms are common. Weather conditions have been found significant in previous studies of injury severity.

The information from Table 5-22 above shows that the proportions of severe injuries were not higher for adverse weather conditions than for dry conditions. The variable in the models combined the slippery or icy conditions to avoid sparse values.

Table 5-23 Traffic control by injury severity for vehicle-driver sections 1 and 2

Driver-vehicle section 1 Driver-vehicle section 2 Traffic

Percent 93.59% 6.41% 120421 100.00

%

Percent 94.34% 5.66% 127819 100.00%

Test of independence p-value=<.0001, Contingency Coefficien =0.0364 t

Another important road variable is the traffic control at intersections. For multilane high speed roads, the intersections with minor roads are generally controlled by either stop signs or traffic signals. Preliminary analysis in section 4.1.3 showed a trend of increased severe injuries for the urban signalized intersections and rural non-signalized intersections. The statistics in Table 5-23 above show the trends of decreased proportions of severe injuries for the signalized intersections. Meanwhile, the stop controlled intersections exhibited significantly higher

proportions of severe injuries (9.42% and 8.41% for sections 1 and 2, respectively). In addition, a smaller number of uncontrolled intersections are included with the road segments (without traffic control or speed limit control). These are captured in the intersection models and possible interactions in the final analysis. The comparison between the stop controlled and the no control yields a severe involvement ratio of 1.47. Excluding other factors, there is 1.47 times the chance of a severe injury per driver involvement at stop controlled intersections versus each involvement at segments or uncontrolled intersections. The stop controlled involvements resulting in severe injuries represent 12% of the total severe injuries. Improvements at these intersections may have a large potential benefit.

After reviewing the exploratory analysis results, it was suspected that crashes reported as yield control in multilane high speed were highly correlated to a traffic signal (right turn lane yield). In a sample from one year of crash data from 5 arterial corridors in two different counties;

almost 50% of the crashes were in fact located at a right turn lane at a signalized intersection, as shown in Table 5-24 below. The sample was deemed acceptable given the fact that yield control crashes represent less than 2% of the total crashes. In addition, flashing beacon control was found to have a correlation with the stop control crashes. Therefore, these two cases were grouped in the traffic control variable.

Table 5-24 Traffic control observed for a sample of crashes recorded as yield control (N=49) Type of traffic control Percent

Traffic Signal 48.98%

Yield at ramp 12.24%

Yield at median 16.33%

Stop Control 4.08%

Yield in minor road 8.16%

Not applicable 10.20%

Total 100.00%