A peer-reviewed open-access journal
Nature Conservation 47: 283–302 (2022)
doi: 10.3897/natureconservation.47.72321 RESEARCH ARTICLE
https://natureconservation.pensoft.net Launched to accelerate biodiversity conservation
Implementing wildlife fences along highways 
at the appropriate spatial scale: A case study of 
reducing road mortality of Florida Key deer
Marcel P. Huijser1, James S. Begley2
1 Western Transportation Institute, Montana State University, PO Box 174250, Bozeman, MT 59717-4250, 
USA 2 Washington Conservation Science Institute, 3414 N Verde St, Tacoma, WA 98407, USA
Corresponding author: Marcel P. Huijser (mhuijser@montana.edu)
Academic editor: Clara Grilo  |  Received 29 July 2021  |  Accepted  13 November 2021  |  Published 25 March 2022
http://zoobank.org/421DFC9F-3DB2-4A09-AD05-43F1E225F3F8
Citation: Huijser MP, Begley JS (2022) Implementing wildlife fences along highways at the appropriate spatial scale: A 
case study of reducing road mortality of Florida Key deer. In: Santos S, Grilo C, Shilling F, Bhardwaj M, Papp CR (Eds) 
Linear Infrastructure Networks with Ecological Solutions. Nature Conservation 47: 283–302. https://doi.org/10.3897/
natureconservation.47.72321
Abstract
Florida Key deer mortality data (1966–2017) showed that about 75% of all reported deer mortalities 
were related to collisions with vehicles. In 2001–2002, the eastern section of US Hwy 1 on Big Pine Key 
(Florida, USA) was mitigated with a wildlife fence, 2 underpasses, and 4 deer guards. After mitigation, the 
number of reported Key deer road mortalities reduced substantially in the mitigated section, but this was 
negated by an increase in collisions along the unmitigated section of US Hwy 1 on Big Pine Key, both in 
absolute numbers and expressed as a percentage of the total deer population size. The data also showed 
that the increase in Key deer collisions along the unmitigated highway section on the island could not be 
explained through an increase in Key deer population size, or by a potential increase in traffic volume. The 
overall Key deer road mortality along US Hwy 1 was not reduced but was moved from the mitigated sec-
tion to the nearby unmitigated section. Thus, there was no net benefit of the fence in reducing collisions. 
After mitigation, a significant hotspot of Key deer-vehicle collisions appeared at the western fence-end, 
and additional hotspots occurred further west along the unmitigated highway. Exploratory spatial analyses 
led us to reject the unmitigated highway section on Big Pine Key as a suitable control for a Before-After-
Control-Impact (BACI) analysis into the effectiveness of the mitigation measures in reducing deer-vehicle 
collisions. Instead, we selected highway sections west and east of Big Pine Key as a control. The BACI 
analysis showed that the wildlife fence and associated mitigation measures were highly effective (95%) in 
reducing deer-vehicle collisions along the mitigated highway section. Nonetheless, in order to reduce the 
overall number of deer-vehicle collisions along US Hwy 1, the entire highway section on Big Pine Key 
Copyright Marcel P. Huijser & James S. Begley. This is an open access article distributed under the terms of the Creative Commons Attribution 
License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source 
are credited.
284 Marcel P. Huijser & James S. Begley  /  Nature Conservation 47: 283–302 (2022)
would need to be mitigated. However, further mitigation is complicated because of the many buildings 
and access roads for businesses and residences. This case study illustrates that while fences and associ-
ated measures can be very effective in reducing collisions, wildlife fences that are too short may result in 
an increase in collisions in nearby unmitigated road sections, especially near fence-ends. Therefore it is 
important to carefully consider the appropriate spatial scale over which highway mitigation measures are 
implemented and evaluated.
Keywords
Collisions, fences, fence-end, key deer, mitigation, net benefit, road ecology, roadkill
Introduction
Most wildlife mitigation measures along highways are aimed at improving human 
safety, reducing direct wildlife mortality, and providing safe crossing opportunities 
for wildlife (e.g. Ford et al. 2009; van der Grift et al. 2017). Fences that are designed 
for large mammals, that are carefully installed and maintained, and that are imple-
mented over at least several miles of road length, can reliably reduce collisions by 
at least 80% (Huijser et al. 2016a; Rytwinski et al. 2016). Since fences alone would 
result in a near absolute linear barrier for the target species, fences are often com-
bined with wildlife crossing structures under or over the road. These underpasses and 
overpasses allow wildlife to safely cross to the other side of the road, and, in general, 
their use increases when they are connected to wildlife fences that help guide the 
animals towards the structures (Dodd et al. 2007; Gagnon et al. 2010). The suit-
ability of the different types of crossing structures (e.g. underpasses vs. overpasses) 
and their dimensions (height, width, length), depend on the species (e.g. Sawyer 
et al. 2016), and sometimes also the sex and age of the individuals (e.g. Ford et al. 
2017). Nonetheless, use of wildlife crossing structures can be considered substantial 
and can increase over time, presumably because the animals learn about the location 
of the structures and that they are safe to use (Clevenger and Barrueto 2014; Huijser 
et al. 2016b).
While a combination of fences and crossing structures is probably the most reli-
able and robust measure to improve human safety, reduce direct wildlife mortality, and 
provide safe crossing opportunities for wildlife, there is still much to learn on how to 
both make fences and crossing structures more effective and have the structures more 
readily accepted by different species (e.g. Huijser et al. 2015a; Rytwinski et al. 2016; 
Denneboom et al. 2021). One of the factors that affects the effectiveness of wildlife 
fences in reducing collisions is the scale at which the fence is implemented. For large 
mammals, at least 5 kilometers (3 miles) of road length needs to be fenced to reliably 
reduce collisions by 80% or more (Huijser et al. 2016a). Collisions that still occur 
within the fenced road sections tend to be concentrated near the fence-ends (Huijser 
et al. 2016b; Plante et al. 2019). Embedding barriers (e.g. wildlife guards or electri-
fied barriers) in the travel lanes at fence-ends, can reduce intrusions into the fenced 
 Implementing wildlife fences along highways at the appropriate spatial scale 285
road corridor (Peterson et al. 2003; Gagnon et al. 2010). However, collisions can also 
be concentrated just beyond the fence-ends in the adjacent unmitigated road sections 
(Huijser et al. 2016b). On a larger spatial scale, there are also some cases where colli-
sions may have been moved further into the adjacent unmitigated road sections (van 
der Grift and Seiler 2016) and where there is no evidence that there was a net benefit 
of wildlife fences. Therefore, it is important to install fences of sufficient length and 
to choose the locations for fence-ends carefully. Fenced road sections should include a 
buffer zone that extends well beyond the known hotspots for wildlife-vehicle collisions 
(Huijser et al. 2015a). Additional considerations such as habitat and topography can 
also help identify suitable locations for fence-ends.
Here we investigate the effectiveness of a wildlife fence and associated measures 
in reducing collisions with an endangered species, the Florida Key deer (Odocoileus 
virginianus clavium), on Big Pine Key, Florida, USA. We first explored the absolute 
Key deer road mortality numbers over the years and evaluated the spatial pattern in 
reported collisions with Key deer before and after the fence was constructed. Then 
we corrected the number of reported collisions for the Key deer population size for 
both the mitigated highway section and different potential control road sections for 
a Before-After-Control-Impact (BACI) analysis through which we evaluated the ef-
fectiveness of the fence. We also investigated potential differences in traffic volume be-
tween the mitigated and unmitigated highway section on Big Pine Key, and how traffic 
volume may have affected Key deer collisions along the unmitigated highway section. 
These exploratory analyses allowed us to find a suitable control for the BACI analysis. 
The careful consideration of different potential control road sections also allowed us to 
explore the net benefit of the wildlife fence on a larger spatial scale. The results help us 
to be more effective when designing wildlife mitigation measures along highways and 
to be more accurate when evaluating their effectiveness.
Methods
Study area and mitigation measures
In 1957, the National Key Deer Refuge was established in the Lower Florida Keys, 
Florida, USA. It is one of four national wildlife refuges in the area. The refuges were es-
tablished to protect the endangered Key deer along with other endangered species and 
the habitat they depend on. At the time, hunting had reduced the Key deer population 
to fewer than 50 individuals (Hardin et al. 1984). Since then, the Key deer popula-
tion has increased to an estimated 1,050 individuals in 2017 (U.S. Fish and Wildlife 
Service 2019). Most of the deer are found from Sugarloaf Key (west) to Big Pine Key 
and No Name Key (east), partially aided by reintroduction on some islands (Parker et 
al. 2008). An estimated 85% of the Key deer occur on Big Pine Key and No Name Key 
(U.S. Fish and Wildlife Service 2019). Parts of the islands have been urbanized, result-
ing in a mosaic of natural habitat (pine rocklands, freshwater marshes, tropical hard-
286 Marcel P. Huijser & James S. Begley  /  Nature Conservation 47: 283–302 (2022)
wood hammocks, transitional buttonwood mangroves, mixed mangrove forests, and 
beach berm communities), residential areas, and commercial lots. This development 
has especially occurred on Big Pine Key. Additionally, tourism has increased substan-
tially over the last several decades (peak season November-April) (Rockport Analytics 
2019; Braden et al. 2020; Key West Travel Guide 2021). This has resulted in more 
than five million visitors per year to the Florida Keys (Rockport Analytics 2019), many 
of whom travel on US Hwy 1 from the mainland of peninsular Florida towards Key 
West. The Average Annual Daily Traffic (AADT) for US Hwy 1 on Big Pine Key was 
about 18,000 vehicles in 2016 (Consulting KBP Inc. 2017). Since there are no natural 
predators for key deer, conflicts with humans, including vehicle collisions, are now the 
most important causes of mortality for the deer (U.S. Fish and Wildlife Service 2019). 
Since 1966, wildlife refuge staff and Florida Fish and Wildlife Conservation Commis-
sion law enforcement staff, aided by other law enforcement staff and the public, have 
recorded Key deer mortalities, including road mortality. Although there is no stand-
ardized monitoring effort for Key deer struck by vehicles, there is high reporting effort 
because of their endangered status and because of the public concern about the species. 
The historic road mortality data showed that most direct road mortality occurred along 
US Hwy 1 on Big Pine Key (Parker 2006), especially in the early morning and early 
evening (Braden et al. 2020). Note that not every dead Key deer and cause of death is 
reported and recorded in the database. This means that there are inherent biases in the 
data; for example, a roadkilled Key deer is more likely to be found and recorded than 
a drowned Key deer.
In 2001–2002 a 1.64 mi (2.64 km) section of US Hwy 1 on the east side of Big 
Pine Key was mitigated with a 2.4 m (8 ft) high fence, 2 underpasses, and 4 deer 
guards (similar to cattle guards) (Braden et al. 2008; Parker et al. 2011) (Fig. 1). US 
Hwy 1 on the west side of the island was left unmitigated because of the access to 
businesses and residential areas (Parker et al. 2008). The mitigated highway section 
is largely situated within natural habitat with only a few access points for side roads. 
The western fence-end and the three side roads all have a deer guard embedded in the 
travel lanes. The eastern fence-end end at the Spanish Harbor Channel Bridge does 
not have a deer guard. Based on a Before-After comparison in a previous study, the 
mitigation measures along US Hwy 1 reduced Key deer collisions by about 90% in 
the mitigated road section (Parker et al. 2011). Furthermore, Key deer use the two 
underpasses, and the use has been increasing with the age of the structures (Braden 
et al. 2008; Parker et al. 2011). However, Key deer collisions continued to increase 
overall, i.e. on other unmitigated road sections (Parker et al. 2011). The continued 
increase in Key deer-vehicle collisions was attributed to the growing Key deer popula-
tion size and traffic volume, especially on Big Pine Key and US Hwy 1 (Parker et al. 
2011). While hurricane Irma blew over large sections of the wildlife fence along US 
Hwy 1 in September 2017, this did not affect our study as we only included Key deer 
mortality data through 2016 for our evaluation of the effectiveness on the measures 
in reducing collisions (see later). Other mitigation measures aiming at reducing col-
lisions with Key deer along both the mitigated and unmitigated section of US Hwy 
1 on Big Pine Key include low maximum posted speed limits (daytime 45 MPH; 
 Implementing wildlife fences along highways at the appropriate spatial scale 287
Figure 1. The mitigated section of US Hwy 1 on Big Pine Key.
nighttime 35 MPH), mobile speed radar units informing drivers of the speed of their 
vehicle, parked police cars (no law enforcement personnel present), and a variety of 
warning and informational signs.
Key deer road mortality numbers and spatial patterns
We used the existing database on Key deer mortalities between 1966 (first record 9 
March 1966) through partway 2017 (last record 9 November 2017) to assess road 
mortalities versus mortality from other causes. We calculated the absolute number of 
Key deer road mortalities along all roads combined (1966–2016) and in the ten years 
before mitigation (1991–2000) and in the fourteen years after mitigation (2003–
2016) along both the mitigated and unmitigated highway section on Big Pine Key. 
We also explored where Key deer collisions occurred before and after the mitigation 
measures were implemented. Exploration of the spatial pattern of Key deer road mor-
talities after the mitigation measures were implemented allowed us to identify loca-
tions where further efforts to reduce Key deer-vehicle collisions should be directed, 
should one choose to do so. In addition, the spatial patterns in Key deer collisions 
before and after the mitigation measures were implemented provided the first step in 
identifying a suitable control for a BACI analysis to calculate the effectiveness of the 
fence and associated measures.
288 Marcel P. Huijser & James S. Begley  /  Nature Conservation 47: 283–302 (2022)
We investigated where the highest greatest concentrations of Key deer roadkill 
were after the eastern section of US Hwy 1 was mitigated. For this hotspot analysis, 
we only selected roadkill records of Key deer for the most recent 10-year period 
(2007 through 2016, n=1,182), regardless of where they occurred i.e. both on and 
off Big Pine Key, both inside and outside the mitigated section of US Hwy 1. We 
chose to use this subset of records as a balance between having recent data that 
identify current hotspots, and having a robust sample size. To identify hotspots, we 
conducted a Kernel density analysis using ArcGIS 10.6.1 (ESRI 2018a) for point 
features of Key deer-vehicle collision locations using a 25 m cell size (82 ft × 82 
ft). A 25 m cell size is relatively fine scale but still accommodates for some spatial 
inaccuracies in GPS coordinates. The Kernel density analysis calculates the density 
of roadkills in a neighborhood around each cell and is based on the quartic kernel 
function described by Silverman (1986). Consistent with Gomes et al. (2009) we set 
the neighborhood search radius at 500 m (0.31 mi). On a straight road this means 
that Key deer roadkill that are up to about 500 m away are included in the density 
analysis for each cell. To help interpret the results of the Kernel density analyses 
and identify hotspots, we displayed the raster output using a heat map classification 
with varying densities of Key deer collisions. We used percentage breaks to create 
five categories (<5%, 5-<25%, 25-<50%, 50-<75%, and 75–100%) that display the 
areas with the highest densities of Key deer collisions (<5%) to areas with the lowest 
densities (75–100%).
Wherever a fence ends, there is a possibility of a concentration of collisions just 
beyond the fence end; the “fence-end effect”. For example, after implementation of 
the fence, some Key deer may have walked alongside the fence until they reached one 
of the two fence-ends. They could then cross the highway at-grade at the fence-end 
where they are exposed to potential collisions with vehicles. If such fence-end effects 
are indeed present, and if such road sections would be included in the control, it 
would result in an overestimation of the collisions in the control section and, through 
the BACI analysis, it would then also overestimate the effectiveness of the mitigated 
road section. Therefore, for a control to be suitable, it should not be influenced by 
the mitigation measures, and potential fence-end effects should be excluded from the 
control. We explored the potential presence of a concentration of Key deer road mor-
tality near the western and the eastern fence-ends through an optimal hot spot analysis 
(Getis-Ord Gi*) in ArcGIS 10.6.1 (ESRI 2018b). This analysis identifies statistically 
significant spatial clusters of hotspots and cold spots of Key deer road mortalities. We 
selected Key deer road mortality observations along US Hwy 1 on Big Pine Key, both 
along the mitigated and unmitigated road section, before and after mitigation, up to 
50 m from the highway. We then created a bounding polygon around the highway 
(50 m buffer from approximately the center of the highway) to allow for some spatial 
imprecision in the original data. We conducted separate analyses for the “before” 
(1991–2000; 331 observations, 5 outliers) and “after” data (2003–2016; 795 observa-
tions, 11 outliers). Within the optimal hot spot analysis (Getis-Ord Gi*) procedure, 
an outlier is defined as a location that is more than a three standard deviation distance 
 Implementing wildlife fences along highways at the appropriate spatial scale 289
away from its closest noncoincident neighbor. For the “before” data, the optimal grid 
size was 43 m, and the optimal fixed distance band was 302 m. For the “after” data, 
the optimal grid size was 44 m, and the optimal fixed distance band was 164 m.
Key deer road mortality in relation to population size
We investigated the net benefit of the mitigation measures by calculating Key deer road 
mortality as a percentage of the Key deer population size for all roads combined, US 
Hwy 1 on Big Pine Key (mitigated and unmitigated sections combined), and the miti-
gated road section of US Hwy 1 on Big Pine Key. We conducted the same analysis for 
different sections of unmitigated road sections of US Hwy 1 to identify a suitable con-
trol for the BACI analysis. The potential control sections that were evaluated included 
the unmitigated road section of US Hwy 1 on Big Pine Key up to the fence-end, the 
unmitigated road section of US Hwy 1 on Big Pine Key excluding a potential fence-
end effect (see previous section), and the unmitigated road sections of US Hwy 1 west 
and east of Big Pine Key. This allowed for a second step in finding a suitable control 
for the BACI analysis as the analyses described above can detect evenly distributed in-
creases in road mortality in different potential control road sections that are associated 
with the implementation of the mitigation measures.
To express Key deer road mortality as a percentage of the Key deer population size, 
we relied on historical population estimates. Unfortunately, total Key deer population 
size estimates were only available for certain years (Appendix 1). The respective authors 
usually presented both a minimum and maximum population estimate. Therefore, we 
calculated the average population size for each of the available minimum and maxi-
mum population estimates. We then fitted an exponential growth curve through the 
available population size estimates, allowing us to calculate the associated population 
size estimate for each calendar year before (1991 through 2000) and after mitigation 
(2003 through 2014). Note that we did not calculate population estimates after 2014, 
the last year the population was estimated based on field work, as we did not want 
to extrapolate beyond the data collection period. We tested for potential differences 
between the percentage of roadkilled Key deer of the total population size in the years 
before and after the mitigation measures were implemented (Kruskal-Wallis One-Way 
ANOVA on Ranks).
Traffic volume
To investigate if traffic volume may have played a role in the increase in Key deer road 
mortality along the unmitigated section of US Hwy 1 on Big Pine Key after the miti-
gation measures were implemented, we summarized traffic volume on US Hwy 1 on 
Big Pine Key between 1994–2017 based on traffic counter data (URS 2017; FDOT 
2018). We tested for a potential difference in traffic volume before (1994–2000) and 
after (2003–2017) implementation of the fence and associated mitigation measures 
(two-sided t-test).
290 Marcel P. Huijser & James S. Begley  /  Nature Conservation 47: 283–302 (2022)
Effectiveness of the mitigation measures
We investigated the effectiveness of the mitigation measures in reducing collisions with 
Key deer through a BACI analysis. We selected roadkill records of Key deer; 10 years 
before the implementation of the mitigation measures (1991 through 2000), and 14 
years after the implementation of the mitigation measures (2003 through 2016). We 
searched for a suitable control section of US Hwy 1 through the analyses described in 
the sections above. The unmitigated section of US Hwy 1 on Big Pine Key, starting 
immediately adjacent to the fence-end, was not suitable due to a concentration of col-
lisions just beyond the fence-end. Excluding the fence-end effect still did not result in 
a suitable control as Key deer road mortality expressed as a percentage of the Key deer 
population size was still elevated, presumably because of the nearby mitigated road 
section. However, the combined unmitigated road sections US Hwy 1 west (11.7 mi; 
18.8 km) and east (2.7 mi; 4.3 km) of Big Pine Key seemed unaffected by the imple-
mentation of the mitigation measures on Big Pine Key. Therefore, we selected these 
road sections as the control for the BACI analysis (total length for the control was 14.4 
mi; 23.2 km) (Fig. 2). Since there was some spatial imprecision in the original data, 
we included observations of roadkilled Key deer that were up to 50 m from either side 
Figure 2. The mitigated section of US Hwy 1 on Big Pine Key and the two highway sections west and 
east of Big Pine Key that served as the control in the BACI analysis.
 Implementing wildlife fences along highways at the appropriate spatial scale 291
of US Hwy 1. For the BACI analysis, we calculated the number of Key deer roadkill 
records per mile for each calendar year for the control (unmitigated) and the impact 
(mitigated) road section. We calculated the BACI effect based on the mean number of 
roadkilled Key deer per mile per year (μ) in the impact road section and the control 
road section before and after the measures were implemented according to (μcontrol,after 
- μcontrol,before) - (μimpact,after - μimpact,before). In addition, the Key deer roadkills per mile per 
year were transformed (ln(x+0.1)) to make the count variable resemble a normal distri-
bution. This allowed for the investigation of a potential interaction of the before-after 
and control-impact parameters through an ANOVA. Should there be an effect of the 
treatment (i.e. the wildlife fence and the associated mitigation measures), we expected 
the effect to result in fewer collisions rather than more. Hence our ANOVA was a one-
sided test.
Results
Key deer road mortality numbers and spatial patterns
There were 4,753 recorded mortalities of Key deer from 1966–2017. Overall, roadkill 
was the most common recorded cause of mortality (N=3,412, 71.8%), followed by 
“undetermined” (N=681, 14.3%), and disease (N=276, 5.8%). Drowning, predation 
by dogs, entanglement, intraspecies combat, poaching, humans (various causes), and 
physical impact of hurricanes each represented less than 5% of the recorded mortalities. 
Road mortality has consistently been the leading known cause of mortality since record 
keeping began in 1966. The average percentage of Key deer road mortalities out of all re-
corded Key deer mortalities for each year (1966–2016) per year was 75.5% (SD=10.2). 
While the absolute number of recorded Key deer road mortalities dropped substan-
tially in the mitigated road section after the mitigation measures were implemented in 
2001–2002, the number of Key deer road mortalities for all roads combined and for 
the unmitigated section of US Hwy 1 on Big Pine Key continued to increase (Fig. 3).
After the mitigation measures were implemented, Key deer road mortality was 
concentrated along the unmitigated western section of US Hwy 1 on Big Pine Key 
(Fig. 4). There were two main hotspots; one on the west side of Big Pine Key (oppo-
site of the canals (W. Cahill Ct.) until Deer Run Tr.), and one at the west end of the 
wildlife fence (opposite of the St. Peter Catholic Church), extending further west till 
Cunningham Ln. Post-mitigation, there were 575 reported Key deer road mortalities 
in the unmitigated section of US Hwy 1, and 25 in the fenced section.
Before the eastern section of US Hwy 1 was mitigated, there was a significant 
concentration of Key deer-vehicle collisions at the eastern edge of Big Pine Key (Fig. 
5a). This hotspot disappeared after the implementation of the mitigation measures, 
and almost the entire length of the mitigated road section turned into a significant 
cold spot (Fig. 5b). However, after mitigation, a significant hotspot appeared at the 
western fence-end, extending for about 300 m (984 ft) into the unmitigated high-
292 Marcel P. Huijser & James S. Begley  /  Nature Conservation 47: 283–302 (2022)
Figure 3. The number of Key deer road mortalities per year along all roads combined (1966–2016) and 
during the ten years before (1991–2000) and the fourteen years after mitigation (2003–2016) along both 
the mitigated and unmitigated section of US Hwy 1 on Big Pine Key (BPK).
way section (Fig. 5b). Other significant hotspots were present further west along 
the unmitigated section of US Hwy 1. The 90% confidence hotspot extended about 
325 m (1,066 ft) from the western fence-end (142 records from 2003–2016; 18.7% 
of the Key deer road mortalities on the unmitigated highway section on Big Pine 
Key). There were 617 records (81.3%) outside this hotspot along the unmitigated 
highway section on Big Pine Key. The 95% confidence hotspot (119 records from 
2003–2016; 15.7% of the Key deer road mortalities on the unmitigated highway 
section on Big Pine Key) extended about 280 meters (919 ft) from the western fence-
end. There were 640 records (84.3%) outside this hotspot along the unmitigated 
highway section on Big Pine Key.
Key deer road mortality in relation to population size
Key deer population size has grown exponentially since the 1940s (Fig. 6). While 
there were only seven population size estimates available in total, the population may 
have been stable or experienced a slight decline between 1974 and 1990. Nonethe-
less, in general, and specifically since the mitigation measures were implemented in 
2001–2002, the population size has grown exponentially.
 Implementing wildlife fences along highways at the appropriate spatial scale 293
Figure 4. Kernel density hotspot map using percentiles for Key deer-vehicle collisions (2007–2016).
Figure 5. Significant hotspots and cold spots for Key deer-vehicle collisions along US Hwy 1 before (a) 
and after (b) mitigation. Numbers represent the mile reference posts.
294 Marcel P. Huijser & James S. Begley  /  Nature Conservation 47: 283–302 (2022)
Figure 6. Estimated Key deer population size. An exponential growth curve (y=e(0.04279*((year)-1853.976); R2 = 
0.93) was fitted through data obtained from the literature (see Appendix 1).
The percentage of roadkilled Key deer of the total population size for all roads com-
bined was similar before (average 14.2%, SD=3.2) and after (average 14.9%, SD=1.8) 
the fence and associated mitigation measures were implemented along the eastern sec-
tion of US Hwy 1 (Kruskal-Wallis One-Way ANOVA on Ranks, Chi2 = 0.109, p = 
0.742) (Fig. 7). The percentage of roadkilled Key deer of the total population size for 
US Hwy 1 on Big Pine Key was also similar before (7.6%, SD=2.1) and after (7.5%, 
SD=1.8) mitigation (Chi2 = 0.017, p = 0.895). However, there was a substantial decrease 
(90.0%) in the mitigated section (before (3.3%, SD=1.2), after (0.3%, SD=0.2)) (Chi2 
= 15.652, p < 0.001). At the same time, there was a substantial increase (68%) in the 
unmitigated section on Big Pine Key (before (4.3%, SD=1.1), after (7.2%, SD=1.7)) 
(Chi2 = 14.126, p < 0.001). There was still an increase in the unmitigated section (65%) 
on Big Pine Key when the 90% confidence hotspot at the fence-end was excluded (be-
fore (3.5%, SD=0.9), after (5.8%, SD=1.5)) (Chi2 = 12.678, p < 0.001). For the unmit-
igated highway section west and east of Big Pine Key there was no significant difference 
before (1.2%, SD=0.6) and after (1.4%, SD=0.6) mitigation (Chi2 = 0.626, p = 0.429).
Traffic volume
In 2017, US Hwy 1 on Big Pine Key had an Annual Average Daily Traffic Volume 
(AADT) of 18,590–19,600 vehicles per day depending on the location of the traffic 
counter (FDOT 2018). The vast majority were passenger cars (92.2%) and 7.8% of 
the vehicles were trucks (single unit, combination trailer, and multi-trailer trucks com-
 Implementing wildlife fences along highways at the appropriate spatial scale 295
Figure 7. The average percentage (and SD) of roadkilled Key deer of the total estimated population size 
in the years before and after implementation of the mitigation measures along the eastern section of US 
Hwy 1 on Big Pine Key (BPK).
Figure 8. Annual Average Daily Traffic (AADT) on US Hwy 1, Big Pine Key (URS 2017; FDOT 2018).
bined) (FDOT 2018). The average and median AADT before mitigation (1994–2000) 
was higher (mean = 20,799; median = 21,186; SD = 1,600) than after mitigation 
(2003–2016) (mean = 18,450; median = 18,053; SD = 1,310) (two-sided t-test t(19) 
= -3.6035, p = 0.002) (Fig. 8).
296 Marcel P. Huijser & James S. Begley  /  Nature Conservation 47: 283–302 (2022)
Effectiveness of the mitigation measures
Before the mitigation measures were implemented, the average number of Key deer 
roadkill per mile per year was 8.4 for the mitigated road section, and 0.4 for the con-
trol section (Fig. 9). After the implementation of the mitigation measures, Key deer 
roadkill decreased by 95.0% in the mitigated road section to 0.5 Key deer roadkilled 
per mile per year. After the implementation of the mitigation measures Key deer road-
kill increased by 112.0% in the control section to 0.8 Key deer roadkilled per mile 
per year. The BACI effect was 8.8; there were nearly 9 fewer roadkilled Key deer per 
mile per year in the mitigated road section when corrected for what happened in the 
control section. In the context of the BACI analysis, the percentage reduction in Key 
deer-vehicle collisions in the mitigated road section was 94.8%. The interaction of 
the before-after and control-impact parameters was significant (one-sided ANOVA 
F1,44=46.63, p < 0.001). This meant that the effect of time (before-after) on the number 
of roadkilled Key deer indeed depended on the implemented mitigation measures.
Figure 9. The average number (and SD) of Key deer-vehicle collisions per mile per year in the control 
and mitigated road section before and after the mitigation measures were implemented.
 Implementing wildlife fences along highways at the appropriate spatial scale 297
Discussion
Key deer road mortality and spatial patterns
Direct road mortality has consistently been the most common recorded cause of mor-
tality for Key deer since record keeping began. Therefore, if the objective is to reduce 
unnatural mortality for Key deer, reducing direct road mortality should be explored 
first. After the mitigation measures were implemented, Key deer road mortality was 
concentrated along the unmitigated western section of US Hwy 1 on Big Pine Key. 
Therefore, this is the road section that should be prioritized if the objective is to reduce 
direct road mortality of Key deer.
Effectiveness of the mitigation measures
Based on the BACI analysis, the wildlife fence and associated mitigation measures 
along the eastern section of US Hwy 1 on Big Pine Key were highly effective (94.8%) 
in reducing Key deer-vehicle collisions along the mitigated road section. However, 
when corrected for the population size, Key deer road mortality was similar before and 
after highway mitigation for all roads combined as well as for US Hwy 1 on Big Pine 
Key (mitigated and unmitigated section combined). Similar to the absolute numbers, 
the percentage of roadkilled Key deer in relation to the population size sharply de-
creased by 90.0% in the mitigated section of US Hwy 1 but substantially increased by 
68% in the unmitigated section of US Hwy 1 and by 65% when the fence-end effect 
was excluded. The hypothesis that the continuing increase in Key deer-vehicle colli-
sions after the mitigation measures were implemented may have been associated with 
an increase in Key deer population size must be rejected. Similarly, traffic volume can 
also not explain the increase in collisions. Traffic volume was, on average, lower after 
the implementation of the fence and associated mitigation measures, likely because of 
the lead-up to the economic crisis in 2008 and the gradual recovery afterwards. How-
ever, in general, higher traffic during certain hours of the night is positively correlated 
with an increase in collisions with Key deer (Braden et al. 2020). Our data suggest that 
while the mitigation measures reduced collisions substantially in the mitigated road 
section, the overall Key deer road mortality on US Hwy 1 on Big Pine Key was not re-
duced. Instead, it was moved from the mitigated section to the unmitigated section of 
US Hwy 1, especially just beyond the fence-end. After mitigation, a significant hotspot 
of Key deer-vehicle collisions appeared at the western fence-end of the mitigated sec-
tion of US Hwy 1, likely as a result of some Key deer following the fence and crossing 
at-grade in higher than average numbers at the fence-end. This is similar to what has 
been observed for other species (Clevenger et al. 2001; van der Grift and Seiler 2016; 
Plante et al. 2019). Other significant Key deer-vehicle collision hotspots after mitiga-
tion occurred further west along the unmitigated highway section on Big Pine Key.
The increase in Key deer road mortality along the unmitigated section of US Hwy 1 
on Big Pine Key can be seen as a form of environmental leakage as the “extraction” was 
298 Marcel P. Huijser & James S. Begley  /  Nature Conservation 47: 283–302 (2022)
moved from a now protected area to a non-protected area rather than reduced (Bode et 
al. 2015). In other words, there was no “net benefit” of the mitigation if the “net ben-
efit” is defined as the gains made in reducing collisions in the fenced road section minus 
the adverse impacts caused by this mitigation, including an increase in collisions in the 
adjacent unmitigated road section (Efroymson et al. 2014).
It is important to bear in mind that the overall number of collisions is just one pa-
rameter associated with the presence of the mitigation measures along the eastern section 
of US Hwy 1. For example, even though the overall number of key-deer vehicle colli-
sions along US Hwy 1 was not reduced after mitigation, the remaining collisions mostly 
occur along the section where the design speed and surroundings (side roads, entrances 
to businesses, pedestrians, cyclists) may encourage drivers to have lower operating speed 
and pay more attention to their surroundings compared to the mitigated section of US 
Hwy 1 (very few side roads, no buildings adjacent to the highway, wide right-of-way). 
Thus, there may be a lower likelihood of human injuries and human fatalities when hit-
ting a Key deer in the western section of US Hwy 1. Another benefit of the mitigation 
measures is that the mitigated section of US Hwy 1 also provides safe crossing oppor-
tunities for Key deer through the underpasses (Braden et al. 2008; Parker et al. 2011).
Management implications
While the mitigation measures along the eastern section of US Hwy 1 on Big Pine 
Key were highly effective in reducing Key deer-vehicle collisions, the data indicate that 
there was no “net benefit” of the wildlife fence in reducing collisions with Key deer 
along the entire section of US Hwy 1 on the island (mitigated and unmitigated road 
section combined). Measures that could be considered for the currently unmitigated 
western road section on the island include erecting a fence behind the businesses and 
residential properties that are adjacent to US Hwy 1. This would mean that the “first 
row” or “first block” of buildings would be included in the fenced road corridor, result-
ing in unhindered access to these buildings from US Hwy 1. A limited number of gaps 
in the fence, with wildlife guards, would allow for access to areas beyond the first row 
or block of buildings. In places where natural habitat remains adjacent to US Hwy 1, 
fenced corridors leading up to US Hwy 1 may be considered for wildlife, including 
Key deer. The fenced corridors would lead to underpasses (similar to the ones in the 
eastern mitigated road section) or at-grade crossing opportunities with wildlife guards 
or electrified barriers embedded in the travel lanes that encourage Key deer to cross the 
highway straight and to keep them from wandering off to the sides into the fenced road 
corridor. This measure can be expected to result in a reduction in Key deer collisions of 
about 95% (with underpasses, see this article) or 40% (with at-grade crossing opportu-
nities, see Lehnert and Bissonette 1997) along the currently unmitigated section of US 
Hwy 1. Alternatively, an animal detection system may be considered, especially at the 
western fence-end. The effectivenes of animal detection systems in reducing collisions 
with large wild animals is extremely variable (33–97%), presumably due to different 
detection technologies, different target species, different types of warning and speed 
 Implementing wildlife fences along highways at the appropriate spatial scale 299
limit reduction signs, and driving culture (see review in Huijser et al. 2015b). None-
theless, an animal detection system along the full length of the 90% probability hot-
spot at the western fence-end would affect 18.7% of the remaining collisions with Key 
deer along US Hwy 1 on Big Pine Key. The fences with two types of crossing opportu-
nities (underpasses, at-grade crossings) and the animal detection system at the western 
fence end, can, depending on the road length along which they are implemented and 
dependent on the spatial distribution of Key deer collisions, all be expected to change 
the current “no net benefit” to a “net benefit” for reducing collisions with Key deer.
Conclusion
In order to substantially reduce the overall number of deer-vehicle collisions along US 
Hwy 1 on Big Pine Key, the entire highway section on Big Pine Key would need to be 
mitigated. However, the section of US Hwy 1 that remains unfenced has many buildings 
and access roads to businesses and residences. This means that there are many compet-
ing interests; implementing mitigation measures that are effective in reducing Key deer-
vehicle collisions and that also provide safe crossing opportunities for Key deer and other 
wildlife species will affect other interests on and along US Hwy 1. This case study also 
illustrates that while fences and associated mitigation measures can be very effective in re-
ducing collisions in the mitigated road section, wildlife-vehicle collisions in the larger area 
may not be reduced because the collisions can move to nearby unmitigated road sections, 
especially just beyond fence-ends. This phenomenon is not an indication that wildlife 
fences do not reduce wildlife-vehicle collisions. Instead, it is an indication that the fenced 
road section is too short. Therefore it is important to carefully consider the appropriate 
spatial scale over which highway mitigation measures are implemented and evaluated.
Acknowledgements
We thank the U.S. Fish & Wildlife Service for funding this project. Special thanks 
are due to Daniel Clark, Christine Ogura, and Kate Watts (all U.S. Fish & Wildlife 
Service) and to Fernando Ascensão, Edgar van der Grift, Clara Grilo, and Bethanie 
Walder for their help and review of different versions of this manuscript.
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Appendix 1
Table 1A. Key deer population size estimates.
Year Minimum (n) Maximum (n) Average (min-max) (n) Source
1940 ? 50 50 Hardin et al. (1984)
1952 25 80 52.5 Dickson (1955)
1974 300 400 350 Klimstra et al. (1974)
1990 250 300 275 Seal and Lacy (1990)
2001 453 517 485 Lopez (2001)
2005 555 619 587 Roberts (2005)
2014 987 1012 999.5 Villanova et al. (2017)