Reducing Vehicle Submersions and Consequent Fatalities on Highway I-75 in Florida (Alligator Alley): Effectiveness of a Cable Safety Barrier System

2.1. The Problem of Vehicle Submersion Fatalities

Each year in Canada and the United States, there are approximately 400 fatalities attributed to vehicle submersions [7, 12] and, moreover, these fatalities account for up to 10% of all accidental drownings [5]. It is especially tragic that most of these fatalities are due to the drowning of occupants who had minimal, or no, trauma [12, 13] and could have survived if they had performed a simple self-rescue. Although many knowledge translation measures are being introduced to inform occupants of what to do in a sinking vehicle, prevention of vehicles reaching water is the best strategy [5].

The mortality rate of crashes into water (7.6%) is about eight times higher than for all types of crashes combined (0.9%); 82% of these deaths were due to drowning [14]. A review of vehicle immersion deaths revealed that the most common cause of vehicles entering water was the loss of control (just under 47%), the fatality rate is much higher in deep water (44%) than in shallow water (5%), and 17% of fatalities occurred in water bodies running alongside the road [5]. In one dangerous canal area in Canada, 19 people drowned over a 52-year period [5]. The Netherlands is the most well known area where canals run beside roadways, and vehicle immersion drownings are prevalent [14]. Thus, the Alligator Alley highway is of particular interest because deep canals run continuously along both sides of the roadway (Fig. 1). One additional risk factor is that the road is long and straight and this might contribute to inattentiveness and fatigue [2], thus increasing the risk of ROR crashes.

2.2. Mitigation of ROR Vehicle Submersions

Multiple strategies and countermeasures have been utilized independently or in combination to decrease the frequency and severity of ROR vehicle crashes.

First, public education on the dangers of vehicle submersion, the promotion of safe driving practices, and the instructions for a proper self-rescue protocol would be valuable [6]. However, a recent study indicated that public understanding regarding vehicle submersion accidents is low, and it was proposed that the simple awareness of the SWOC self-rescue protocol [e.g., SEATBELTS off, WINDOWS open or broken, OUT immediately, CHILDREN first (oldest to youngest)] could help decrease the high fatality rate [15].

Second, warning systems can alert drivers about dangers near or alongside roadways such as: roadway signage (e.g., bridge/curve ahead and potential for flooding); and other warning methods (e.g., rumble strips, markings/reflectors, and weather warnings) in areas of increased risk [7-9]. Third, road and vehicle designs (e.g., road curvature, lighting, speed limits, skid resistant pavement, electronic stability control, lane departure technologies, and anti-lock braking) help decrease the potential for an ROR accident [2].

Finally, spatial or physical barriers are used to either prevent vehicles from leaving the roadway, or impacting secondary structures after leaving the roadway. Spatial barriers simply provide a large space between the road and secondary objects (e.g., structures or water bodies) [16]. Physical barriers can also prevent, or minimize the effect of secondary impact. They are classified as either rigid (e.g., concrete barricades) [7, 17] or pliant (e.g., sand or water-filled barrels, metal guardrails and cable barriers) [18].

2.3. Effectiveness of Crash Mitigation Efforts

Effectiveness of many mitigation systems have been analysed. For example, in one study, automated speed enforcement cameras and chevron signs have been shown to decrease traffic crashes, while delineator posts and rumble strips have not [19]. Other studies have shown positive results with rumble strips [20-22] and other systems such as transition road safety barrier systems (e.g., metal guard rails) [17].

Rigid (concrete) and pliant (metal and cable) barriers have been installed in medians to prevent crossover crashes, and along the roadside to prevent ROR crashes. Pliant barriers, especially cable systems, have the advantage of absorbing more kinetic energy and preventing vehicles from crossing into oncoming traffic, or rebounding back into the roadway from which they left [18, 23].

In general, barriers have been shown to reduce the percentage of total crashes that result in casualties by 40-70% [24-26]. Metal and cable barriers installed in medians have been shown to reduce or eliminate crossover crashes and decrease injury crashes by 29-35% [27]. Specifically, median CSB systems have been shown to decrease head-on crashes by 96%, fatal crashes by 82% and incapacitation injury crashes by 76% [28].

The use and evaluation of CSB systems installed lateral to the roadway is limited. Although roadside barriers have decreased general injury crashes by 32-48% [27], we are unaware of research on the effects of CSB systems in the prevention of ROR vehicle submersion deaths. Thus, the current analysis was conducted for Alligator Alley, a highway section that had an extremely high rate of vehicle submersion deaths.

3.1. Data Collection

Data from before and after the installation period (2003-2004) were collected from four sources: 1) FDOT (and police reports when available); 2) the Fatal Analysis Reporting System (FARS); 3) media reports supplied by FDOT; 4) and media reports from on-line searches.

In the State of Florida, law enforcement agencies complete crash reports for traffic crashes which are housed by Florida Highway Safety and Motor Vehicles (FLHSMV). Crash data for all traffic crashes are provided to the FDOT Crash Analysis Reporting database [29]. As well, data for all fatal traffic crashes on public roadways are provided by FLHSMV to the National Highway Traffic Safety Administration (NHTSA) for inclusion in the FARS national database [30].

First, pre-installation data from FDOT was limited for the following reasons. The FDOT converted to a digital data collection system in 2007 and pre-2007 data were not digitized. These pre-2007 paper files were lost through a storage accident and were no longer available. However, the summary data used by FDOT for their pre-installation study had been extracted and stored elsewhere. Therefore, limited mean data were available for the 1995-1999 period. Post-installation data were analysed from a similar 5-yr period from 2007-2011; this data included police reports. Thus, the limited pre-installation data includes total accidents, the number of vehicles reaching the water, and vehicle immersion fatalities. Post-installation data also included direction of travel for ROR vehicles and vehicles hitting the cable system; police reports added information as to whether vehicles penetrated or overrode the fence, and the final vehicle position.

Second, more complete data were available from the FARS database; thus data were collected for 8-yr periods before (1995-2002), and after (2005-2012) the CSB installation. The FARS database was searched to target ROR vehicle immersions on I-75 in Collier County. The variables and search criteria were as follows (code): State, Florida (3); County, Collier (021); First Harmful Event, Any (All); Number of Fatalities, Any (All); and Most Harmful Event, Vehicle Immersion (3). In the FARS system, all vehicle immersions are classified with the most harmful event being Vehicle Immersion (VI). All resultant cases were then manually evaluated and were only included when the accident occurred on I-75 (based on the traffic identifier in the FARS system). This provided confirmation on the number of yearly fatalities in ROR vehicle immersions on Alligator Alley in Collier County and also provided the first harmful event for all of these cases.

Third, upon request, the FDOT provided a copy of all reports and media they had on hand regarding the CSB system on Alligator Alley. Finally, an online search was conducted for any media coverage for accidents on Alligator Alley between 1995 and 2014. Media sources were compiled by years and if possible, paired to the corresponding FARS case reports. This was done to match and capture any additional information regarding crashes occurring in this timeframe.

3.2. Data Analysis

Only descriptive statistics are presented for FDOT data since they were only available as single mean values for the 5-yr pre-installation period (1995-1999) and accident descriptions were only available the post-installation period (2007-2011).

Since yearly FARS data were available, unpaired t-tests were used to compare mean values for the number of vehicle immersions and vehicle immersion fatalities, for the 8-yr pre-installation (1995-2002) and post-installation (2005-2012) periods. Data were analyzed with the SigmaStat package within SigmaPlot 14. Significance for all analyses was set at P ≤ 0.05. For each pre-and post-installation period, a linear regression analysis was conducted for each variable (from FARS). If a significant correlation was detected in the pre-installation period, the trend could be used to predict values for the post-installation period if the CSB system had not been installed.

4.1. History of Alligator Alley

Alligator Alley was originally designed as a 2-lane undivided roadway. It opened in 1968 as a straight east-west toll-way connecting southern Florida through the everglade wetlands between Naples and Fort Lauderdale. It traverses Collier (80 km) and Broward (40 km) counties. This roadway quickly became well known for high-speed accidents, including head-on collisions and collisions with wildlife [11].

To increase safety and address growing traffic volumes, several upgrades were made. By 1992 Alligator Alley was a 4-lane divided highway. Notably, drainage canals (up to 12 m deep) ran parallel to both sides of the highway. A minimum clear zone of 18.3 m formed a spatial barrier between the road and canals to decrease the likelihood of vehicles reaching and entering the water [31] (Fig. 1). Additionally, chain-link wildlife fencing (FDOT Standard Type A and B) along the canals restricted animal access to the roads.

The evaluation indicated that, due to high speeds on the straight roadways, many ROR vehicles still traversed the spatial barriers and entered the canals. Importantly, the wildlife fencing did not act as a barrier for ROR vehicles, but instead actually increased the difficulty in, and response time for locating the crash sites since the fencing could return to its original position after the vehicle passed under or over the fencing. The FDOT suspected many of the occupants in ROR vehicles that reached the canals would have survived their crash-related injuries, and may not have drowned in the canal had they been found and rescued sooner [32].

In 2002, the FDOT began a process to implement a new safety intervention after 26 vehicle submersions occurred in a single year in the canals along Alligator Alley, resulting in six drowning fatalities. A novel cable safety barrier system was selected by the FDOT to be integrated with the existing wildlife fencing along Alligator Alley in Collier County. This unique system was proposed to 1) minimize the effects of an ROR accident (i.e., injury, fatality and property damage); 2) prevent ROR vehicles from reaching and entering the canals; 3) facilitate faster detection and response times for emergency services for ROR crashes; and 4) maintain and protect the natural beauty of the Florida Everglades [32].

4.2. Description of Cable Safety Barrier System

The system was constructed from 2003-2004 at a total cost of 4.2 million dollars USD. This was the first installation of a CSB system in Florida. It used two 19 mm diameter galvanized steel cables installed 53 cm and 69 cm above the ground. The existing wildlife fence posts support these cables. Sections of the cable fencing are contiguous except for areas where culverts cross under the highway. Guardrails are installed over these culvert areas close to the road shoulder. The CSB extends well past the beginning of the guardrail but does not start again until after the guardrail ends. CSB sections are anchored by foundation systems spaced no more than 610 m apart. Terminal foundation systems anchor the terminal sections of the CSB system (e.g., at the beginning and end of guardrails). The remaining non-terminal foundation systems anchor any two adjoining CSB sections (Fig. 2). Each cable section attaches to an underground anchor (concrete Jersey barrier) within the foundation system via heavy-duty springs providing 907 kg of tension (Figs. 2 and 3), with a breakaway switch (Fig. 4) attached to the top cable. Therefore, cable impact stretches the spring and triggers the breakaway switch; this activates a strobe light and signals a regional traffic management centre to indicate the approximate location of the collision. As well, on impact, the slack in the cable system is taken up to safely decelerate and capture errant vehicles. This system not only prevents vehicles from entering the canals, but also reduces injury severity that might occur if a solid (e.g., concrete) barrier was used.

5.1. Performance by Rates of Vehicle Immersions and Fatalities

Table 1 provides annual accident and fatality rates for Alligator Alley in Collier County as determined from the FDOT and FARS data.

As stated above, FDOT data for the pre-installation period (1995-99) were only available in aggregate form; therefore statistical analysis could not be performed; this data is still instructive, however. There was a rise in overall accident rates from pre- to post-installation of the CSB system [annual total accidents increased from 81.4/y (for 1995-99) to 106.2/y (for 2007-11)]; this corresponds to increased traffic volumes from 1995-2011 [29] and is consistent with predictions such as the surrogate safety assessment model [33]. Comparing the pre- to post-installation periods, the percentage of all accidents resulting in ROR vehicle immersions decreased from 13.8% to 2.4%, and the percentage of all accidents that resulted in vehicle immersion fatalities decreased from 3.4% to 0.4% (FDOT data) (Table 1).

Regression analysis for the FARS data determined that there were no significant correlations or trends for vehicle immersions or vehicle immersion fatalities in either period. Therefore, pre-installation data could not predict what would have occurred post-installation if a CSB system had not been installed. Therefore, the best indicator of the effectiveness of the CSB system was to compare mean data for the pre- and post-installation periods. Statistical analysis of the FARS data indicates that the installation of the CSB system corresponded to a significant decrease for fatal crashes, in both the number of vehicles (from 2.4/y to 0.9/y; P < 0.01), and fatalities (from 3.3 to 1.4/y; P < 0.05) in fatal accidents where vehicle immersion was the most harmful event (MHE) (Table 1). Prior to installation of the CSB system, for fatal vehicle immersions, 84.2% involved vehicles that first experienced a collision with the wildlife fencing. After installation of the CSB system, only 14.3% of fatal vehicle immersions involved vehicles that first experienced a collision with the CSB system (P < 0.01). Notably, total vehicle immersions and vehicle immersion fatalities for the State of Florida were unchanged from the pre- to post-installation periods (Table A1). Therefore, vehicle immersion fatalities on Alligator Alley, as a percentage of total state fatalities, decreased from 7.2% pre-installation to 3.2% post-installation.

5.2. Performance for Barrier Crossing Prevention

Further analysis of post-installation data from FDOT and police records indicates that from 2007 to 2011, there were a total of 531 accidents with 110 ROR vehicles travelling towards the canals (Table 2 mean data, Table A2 yearly data). Ninety-one vehicles contacted the CSB system, with only 14 vehicles (15.4%) penetrating the barrier, and 7 (7.7%) overriding the barrier.

5.3. Summary of Results

Following the installation of the CSB system, there was a decrease in the proportion of total accidents that resulted in vehicle immersions (from 13.8% to 2.4%) and vehicle immersion fatalities (3.4% to 0.4%) (FDOT). This positive outcome is consistent with the decrease in the rate of fatal vehicle immersions occurring after hitting the wildlife fences (84.2%) compared to hitting the cable barrier (14.3%) (FARS).