Safety and NEPA: Case Studies and Noteworthy Practices

 

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Themes featured in this case study:

Considering Safety Prior to NEPA

Public and Stakeholder Engagement

Safety in Purpose and Need

Safety in Mitigation

FHWA-SA-22-044

Colorado’s I-25 

South Gap 

Environmental Assessment

Introduction

Interstate 25 (I-25) is the only north-to-south interstate route 

in Colorado, connecting the State's largest population centers, 

providing access to recreation and cultural amenities, and 

accommodating $60 billion of freight movement annually. 

Informally known as “The Gap,” the I-25 project corridor is 

an 18-mile stretch from the Town of Castle Rock in the north

to the Town of Monument in the south. Providing only two 

travel lanes per direction, the Gap creates a bottleneck for

I-25 travel as it is the only four-lane section of I-25 between 

Denver and Colorado Springs. The corridor links major urban 

areas, but it is also valued for its protected open spaces, 

scenic vistas, and rural setting.

At an elevation of 7,352 feet (ft), the crest of Monument Hill 

near the southern limits of the Gap corridor is the highest 

point on I-25 through Colorado. Grades steadily climb in the 

southbound direction of the corridor, where over the course 

of approximately 15 miles, the elevation increases 

approximately 1,000 ft. Over the years, congestion, crashes, 

and delays have increased due to population and traffic

growth. Improving I-25 through the Gap became a top 

statewide priority, and this case study presents how the 

Colorado Department of Transportation (CDOT) was able to 

incorporate safety as part of the NEPA process to rapidly 

address a major transportation need for the State (figure 1).

Figure 1. Steps of the NEPA process covered by this case study.

Source: FHWA
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Project Background and Timeline

The Gap was part of a broader 34-mile 

planning and environmental linkages (PEL) 

study initiated by CDOT in summer 

2016. At the beginning of the PEL Study, 

CDOT did not have funding identified for 

corridor improvements. However, a need 

for safety, mobility, incident management,

and travel reliability improvements for 

the I-25 South Gap segment between 

Monument and Castle Rock (figure 2) 

became apparent to CDOT staff. Funding 

became available as public and 

stakeholder interest grew, including 

political interest from the State’s 

governor, as well as Federal and State 

lawmakers. 

Available funding from the State of 

Colorado and CDOT’s funding partners 

(Douglas County, El Paso County, and the 

Pikes Peak Rural Transportation Authority), 

prompted CDOT and the Federal Highway Administration (FHWA) to accelerate survey, design, and detailed 

environmental studies for the Gap improvements to occur concurrently with the broader PEL Study. These 

detailed studies started in the summer of 2017. This timing was essential to initiate construction in the fall of

2018 before the onset of poor winter weather common in the Gap.

The PEL study identified this part of the 34-mile corridor as having the most severe safety issues that needed 

immediate attention. CDOT progressed the design for the I-25 South Gap Project based on conceptual 

improvements identified in the PEL Study. The following timeline represents the key project milestones (figure 

3). 

Figure 2. Graphic. I-25 project study area. 

Source: CDOT, 2018. 
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Project Summary 

The I-25 South Gap EA incorporates elements of safety in each stage of the NEPA process. The following 

sections document how CDOT considered safety in relation to other project needs.

Safety in Pre-NEPA Studies

CDOT, in cooperation with FHWA, conducted a PEL study to establish a long-term vision and strategic plan 

for future transportation improvements on a 34-mile segment of the I-25 corridor between Monument (State 

Highway 105) north to the interchange with Colorado Highway 470 (C-470) and E-470 (C/E-470). The PEL 

study identified transportation priorities in advance of securing construction funding, positioning CDOT to 

accelerate the environmental analyses and expedite project implementation when construction funds became 

available. The PEL study laid the groundwork for future improvements on I-25 by:

» Defining and prioritizing projects in the corridor. 

» Determining project costs, funding, financing, and delivery options. 

» Engaging with local corridor communities, regional travelers, and other interested stakeholders about 

corridor issues and priorities. 

» Identifying significant environmental constraints that may influence design options and/or delay project 

development with lengthy environmental reviews. 

» Supporting an efficient transition to the NEPA processes, final design, and construction. 

The PEL study followed FHWA and CDOT guidance (CDOT, 2022)1 regarding the integration of 

transportation planning and the NEPA process; this guidance encourages the use of planning studies to provide 

1 Note that CDOT completed the I-25 PEL study using a previous version of the guidance, Version 2.

Figure 3. Graphic. I-25 South Gap project development process.
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information for incorporation into future NEPA documents.

2 FHWA promotes the use of PELs, largely to 

integrate environmental issues and public involvement with project planning and shorten the time required to 

take projects from planning to implementation.

The four key steps in the PEL study process are described on figure 4. 

2 This guidance builds on 23 U.S.C §168 that enables transportation agencies to integrate “planning products” into environmental 

review. Sections 2.1 (Legal and Regulatory Background) and 2.3 (Legal Requirements) discuss the requirements for a PEL study to be 

incorporated or referenced in NEPA documentation (CDOT, 2022). 

Figure 4. Graphic. Key steps in the PEL study recommendation development process. 

Source: CDOT, 2019.
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During the PEL study, CDOT evaluated safety issues by reviewing the existing infrastructure along the 

corridor and analyzing crash data over the most recent 5-year period for which data were available (2011 

through 2015). CDOT produced a Safety Assessment Report (SAR) that investigated the magnitude of 

historical safety problems on the corridor. This approach used State-specific safety performance functions 

(SPFs) to determine the Level of Service of Safety (LOSS; figure 5). 

The LOSS methodology is one of the network screening performance measures described in the American 

Association of State Highway Transportation Officials’ (AASHTO) Highway Safety Manual (HSM; 2010). LOSS

compares a roadway segment’s observed crash frequency and severity to the average crash frequencies and 

severities predicted for similar sites using an SPF (i.e., the typical number of crashes one might expect for a 

segment or intersection with similar characteristics). There are four LOSS categories that indicate the 

potential for crash reduction:

» LOSS-I – Indicates low potential for crash reduction. 

» LOSS-II – Indicates low to moderate potential for crash reduction. 

» LOSS-III – Indicates moderate to high potential for crash reduction. 

» LOSS-IV – Indicated high potential for crash reduction. 

CDOT determined there was a 

moderate to high potential to reduce 

crashes and improve safety with a LOSSIII designation for 16.1 miles of the 

corridor and a high potential to reduce 

crashes and improve safety with a LOSSIV designation for the remaining 1.9 miles

of the corridor. The SAR indicated that 

safety issues on I-25 were primarily 

related to congestion, physical roadway 

conditions (i.e., narrow shoulders, poor 

lighting, steep grades, etc.), wildlife 

vehicle collisions (WVCs), and incident 

management.

Safety in Project Scoping

CDOT conducted extensive outreach with 

corridor communities and stakeholders to 

understand the most pressing travel issues and highest priority improvements through over 30 public 

meetings, hearings, and listening sessions during the PEL study and formal EA scoping. Public involvement 

during the PEL study included 8 public meetings, 3 telephone town halls, and numerous listening sessions.

Public involvement during the EA comprised 2 public meetings and 2 public hearings, although Steering 

Committee meetings during the EA process were open to the public as well. Over the course of the 

combined PEL study and EA phase of the project, CDOT conducted 22 Technical Working Group meetings 

and 16 Regulatory Agency Group meetings along with numerous Steering Committee meetings. The deaths of 

two Colorado State Troopers in the Gap (2014 and 2015) outside of their vehicles while on duty served as a 

Figure 5. Graphic. Example distribution of sites relative to the SPF prediction 

(“mean”) by LOSS. 

Source: FHWA, 2011a. 
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catalyst to jumpstart the safety discussion at many of the stakeholder and public meetings. As a result, the 

stakeholder groups concluded that safety concerns were paramount. 

“Expanding I-25 between Castle Rock and Monument is crucial to meet the national security, public 

safety, and economic needs for southern Colorado in the years to come. Waiting is no longer an option. 

We must move this project forward now”- El Paso County Commissioner Mark Waller (CDOT, 2018).

Coordination with emergency responders identified the lack of ability for responders to reach an incident, as 

well as difficulty transporting injured persons from an incident scene. CDOT quickly identified the need for 

widened shoulders and law enforcement pull-off areas based on focused meetings with emergency responders. 

These recommendations moved forward from the earlier PEL meetings, and CDOT incorporated wider inside 

shoulders into the project’s design.

Safety in Project Purpose and Need 

The project purpose was to enhance safety, reduce delays, and improve travel time reliability on I-25 through 

the 18-mile Gap section. As noted in the EA: “The I-25 South Gap Project is needed to improve safety, reduce 

crashes, and improve incident management; reduce delays; and improve travel reliability on I-25 through the 

Gap” (CDOT, 2018; p. 2-1). CDOT’s safety analysis indicated the entire length of the 18-mile Gap segment 

had the potential for safety to be improved, particularly in the areas of vehicular safety, WVCs, and incident 

response. 

A high number of crashes occurred on the corridor during the study period (2011 to 2015)—averaging one 

each day. The majority of the crashes in the Gap were rear-ends, followed by sideswipe in same direction and 

fixed-object crashes. Rear-ends and sideswipe same direction crashes can be indicative of volatility or 

turbulence in traffic. Many factors contributed to turbulence in the Gap segment, including: 

» Volume demand. 

» Transitions between two- and three-lane segments. 

» Interchange influence areas. 

» Speed changes. 

» Lack of passing opportunities with slower moving vehicles negotiating steep grades. 

» Non-regular weekend drivers who may not be familiar with the corridor’s challenges.
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There are 55 square miles of protected open space on either side of I-25 through the Gap that supports 

healthy herds of deer, elk, and bighorn sheep, as well as robust populations of mountain lions and black bears. 

Wildlife collisions represented 10 percent of the reported corridor crashes (CDOT, 2018). Both carcass 

removal (CDOT Maintenance) and official crash data (CDOT Traffic and Safety Section) noted considerable 

peaks at the south and north ends of the Gap (figure 6). 

As part of the PEL and EA coordination, the project team conducted numerous interviews with emergency 

responders and CDOT Maintenance staff to better understand the severity of WVCs in the Gap. The project 

team learned that potential WVCs were underestimated by at least 50-60 percent based on official records; 

meaning that at least one WVC occurred every day in the spring and fall (heavy movement seasons for the 

herds). 

Safety concerns along the Gap corridor involved primary crashes, but additional issues also included other 

special events, maintenance, speed differentials, weather, and resulting secondary crashes. Due to the difficulty 

of clearing crashes or other disabled vehicles, the Gap experienced increased exposure for secondary crashes 

(i.e., crashes that occur in the congested conditions resulting from an initial crash). This presented safety 

concerns for both disabled vehicle occupants and emergency responders. FHWA estimated the likelihood of a 

secondary crash increases by 2.8 percent for each minute the primary incident continues to be unattended 

(FHWA, 2016b).

Figure 6. Graphic. Locations of reported wildlife collisions in the Gap (2006-2016). 

Source: CDOT, 2018.
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The factors that affected the ability for emergency equipment to reach crashes and for incidents to be cleared 

were narrow paved shoulders (2-4 ft) coupled with guardrails, long distances between interchanges, lack of 

alternative routes, lack of emergency parking, lack of emergency crossovers caused by split grades, and lack of 

closure gates and messaging signs 

warning travelers of incidents. Lane or

full highway closures often resulted as 

crashes were investigated and cleared. 

Highway maintenance workers and law 

enforcement officers faced similar 

safety issues as others responding to

crashes while conducting operations in 

the corridor with narrow shoulders 

next to high-speed interstate traffic. 

Furthermore, the narrow shoulders did

not provide space for through traffic to 

maneuver around crashes, preventing 

emergency responders from reaching 

the scene. Due to the lack of suitable 

shoulders (figure 7), Colorado State 

Patrol (CSP) had limited ability to 

conduct law enforcement operations 

and interactions with drivers; this was 

highlighted by the deaths of two onduty officers prior to the EA. 

Safety in Alternatives Analysis

CDOT initially developed and evaluated alternative concepts in the PEL study process. CDOT evaluated

information and concepts from the PEL study during the EA process, and CDOT refined the alternatives as 

the project advanced through the NEPA process. CDOT formally incorporated safety in the project purpose 

and need; that allowed CDOT to justify alternatives that included design elements which addressed overall 

safety along the I-25 South Gap corridor. Specific elements included widened shoulders, wildlife underpasses, 

bridge replacements, curve realignment, pavement resurfacing, reducing speed differentials by adding a 4-mile 

climbing lane, Dark Skies compliant LED lighting, Intelligent Transportation Systems (ITS) and signage, new 

auxiliary lanes, and other ancillary features. To address the underlying need to increase the two-lane 

bottleneck through the Gap to a three-lane, CDOT considered two alternatives for the operation of the third 

lane (in each direction), a managed express lane and a general-purpose lane. 

Express Lane Alternative

The broader PEL study identified travel time reliability as a key need for the I-25 South Gap Project, and this 

recommendation reinforced the justification for a managed lane alternative. This alternative would add a new 

12-ft tolled express lane with a 4-ft painted buffer to separate the express lane from the existing general 

purpose (i.e., non-tolled) lanes. These would be added in both directions of travel along I-25 through the Gap. 

The express lane would function as a high-occupancy toll (HOT) lane and provide travel reliability for CDOT’s 

regional bus service, Bustang. Travelers would be able to travel in the express lanes by paying a toll, or for 

free if they ride with three or more people (with a switchable transponder). The existing two general purpose 

lanes would remain toll-free in both directions. 

Figure 7. Photograph. Example of split grade and narrow shoulders in the Gap (2006-

2016). 

Source: Attardo, 2020. 
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General Purpose Lane Alternative

This alternative would add a new 12-ft general-purpose lane in each direction along I-25 through the Gap, 

providing 3 general-purpose (i.e., non-tolled) lanes in each direction.

Safety in the Preferred Alternative

The alternatives analysis identified the express lane alternative as the preferred alternative. Although safety 

was not a major deciding factor between either alternative, CDOT added safety-related features to each 

design to address possible concerns. The preferred alternative addressed key safety issues identified in the 

project purpose and need: 

Vehicular Safety: CDOT developed several design decisions that targeted the types of crashes noted 

during the EA process. These improvements included increased capacity, wider shoulders, improved 

drainage, improved signage, pavement markings and delineators, interchange lighting, and lengthened 

acceleration and deceleration lanes. 

WVCs: New wildlife underpasses would provide animals the opportunity to cross under I-25 rather 

than crossing the interstate at-grade. This would decrease the likelihood of a WVC. Additional wildlife 

fencing would promote the usage of the wildlife underpasses. 

Incident Management: The preferred alternative included design elements that would promote safety for 

maintenance, emergency response, and law enforcement workers. Wider shoulders and other features 

would provide room for responders and law enforcement officers to work on the roadside and reduce 

exposure to moving traffic. Likewise, safer, and more efficient incident management should also reduce 

the likelihood of secondary crashes. Although future applications are not yet determined as of this case 

study, the addition of variable message signs (VMS) along the corridor can also be used to alert drivers 

to changing or emergency conditions that should further improve incident management and other 

safety issues.

As a result of the NEPA process, CDOT and FHWA selected a preferred alternative with the following 

features: 

» Six-lane Typical Section (figure 8) 

o Add a 12-ft express lane and a 4-ft buffer between express and general-purpose lanes in both 

directions. 

o Widen outside and inside shoulders to 12 and 15 ft, respectively.

» Rehabilitate structures and pavement, including replacing the I-25 bridges over Plum Creek, Greenland 

Road, and Upper Lake Gulch Road.

» Implement Wildlife Mitigation System

o Provide 4 new wildlife underpasses and expand existing underpass. 

o Provide 28 miles of deer fence, 64 escape ramps, and 21 deer guards. 

» Add median and retaining walls throughout the corridor to keep the improvements within CDOT 

right-of-way. 

» Improve drainage and other features, such as lighting, signage, fencing.

» Open southbound rest area to allow for truck chain-up in inclement weather along with extended 

acceleration and deceleration lanes. 

» Install tolling, communications, and power equipment for driver and vehicle communications.
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Safety, the Affected Environment, and Environmental Consequences

CDOT assessed several elements that could be impacted by the project during the EA process. These 

elements include: 

» Wildlife movement. 

» Threatened and endangered species. 

» Wetland/Waters of the United States.

» Floodplains. 

» Water quality. 

» Paleontological resources. 

» Vegetation and habitat. 

» Noxious weeds. 

» Socioeconomic resources. 

» Visual resources. 

» Recreational resources. 

Although CDOT planned to widen the 18-mile I-25 South Gap corridor, the agency relied on design 

alternatives that stayed within the existing ROW (to the greatest extent possible). Staying within the existing 

ROW helped to reduce the direct impacts to streams, wetlands, parks/open space, and endangered and 

protected species habitats. CDOT mitigated the small impacts that occurred as part of the construction. 

The presence of protected wildlife habitat on both sides of the corridor, lack of crossing opportunities for 

wildlife, and low-light conditions for drivers contributed to the high number of incidents involving wildlife. Due 

Figure 8. Graphic. Existing typical section of the Gap compared to the preferred alternative. 

Source: CDOT, 2018.
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to the high number of WVCs and quality of wildlife habitat in the area, Colorado Parks and Wildlife (CPW) 

identified the Gap corridor as one of the seven “high-risk” locations for WVCs statewide. CDOT developed a

Wildlife Movement Technical Memorandum as part of the project documentation, and the analysis supported 

the investment and construction of a wildlife mitigation system as part of the I-25 South Gap Project.

The preferred alternative addressed the lack of wildlife connectivity and the potential to reduce the number of 

WVCs by adding 4 additional wildlife underpasses at strategic locations and doubling the size of an existing 

crossing from 150 ft to 300 ft. Although the preferred alternative would impact other environmental 

resources as a result of a wider footprint, incorporating wildlife crossings would satisfy a key environmental 

need of the project by potentially reducing wildlife exposure to traffic and therefore the total number of 

WVCs (figure 9). 

The wildlife crossings work together with 8-ft wildlife exclusion fence, deer guards, and escape ramps that 

function together as a Wildlife Mitigation System. CDOT installed wildlife fencing (28 miles) along both sides 

of I-25 to encourage deer and other animals to use the new underpasses and deter them from crossing the 

interstate. CDOT also constructed wildlife escape ramps/jump-outs that would allow animals that enter I-25 

through openings in the fence to escape, and deer guards that would deter animals from entering I-25 where 

gaps in the wildlife fencing are unavoidable (e.g., interchange on-ramps and emergency access points). CDOT 

completed the Wildlife Mitigation System implementation in 2021. A three-year joint CDOT and CPW 

research project is monitoring the system with action triggered cameras. 

Safety in Mitigation, Environmental Commitments, and Post-NEPA

CDOT monitored environmental commitments using a tracking summary sheet in the EA. Documented and 

tracked commitments included safety-related measures such as proposed construction of wildlife underpasses

and installation of wildlife fencing aimed at minimizing WVCs and improving roadway safety. Environmental 

Figure 9. Photograph. Example wildlife underpass. 

Source: Attardo, 2020. 
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related specifications and special provisions were incorporated into the construction documents. 

Environmental monitors tracked and monitored environmental commitments during construction. Monthly 

environmental commitments and mitigation tracking reports were prepared and rolled up into annual reports. 

The final environmental monitoring report noted that CDOT met (i.e., did not exceed) all of its prescribed 

impact levels during the construction process.

Project Documentation

CDOT completed an EA with FHWA approval of the Finding of No Significant Impact (FONSI) in June 2018. 

Once the EA and FONSI were completed, CDOT re-initiated the larger 34-mile PEL study and completed it in 

October 2019.

Project Themes

The I-25 South Gap EA touches on several themes noted in FHWA’s Integrating Road Safety into NEPA Analysis: 

A Practitioner’s Primer guidance (FHWA, 2011b). These themes are also key takeaways for future NEPA studies 

that plan to effectively incorporate safety early in the NEPA and project development processes. 

Considering Safety Prior to NEPA 

CDOT evaluated safety issues prior to NEPA during the initial stages of the PEL study. This included a review

of the existing infrastructure in the corridor and an analysis of crash data during the most recent 5-year period 

for which data were available. CDOT produced a SAR that looked at the magnitude of safety problems on the 

corridor using CDOT’s SPFs and LOSS derived from HSM methodologies. Safety issues on I-25 were primarily 

related to congestion, physical roadway conditions, and incident management, although animal crashes and 

carcass locations helped inform NEPA recommendations. CDOT determined there was a moderate to high 

potential to reduce crashes and improve safety along a majority of the corridor. The PEL study was the first 

step to analyze and address safety, travel reliability and mobility issues along I-25, and laid the groundwork for 

the NEPA process to identify and prioritize the I-25 South Gap Project.

Public and Stakeholder Outreach 

Outreach formed a key mechanism for including safety in purpose and need. CDOT conducted extensive 

outreach with corridor communities and stakeholders during both the PEL study and for the EA to understand 

the most critical travel issues and highest priority improvements. Discussions with law enforcement and 

emergency services also revealed key bottlenecks with the existing design that created safety concerns for 

responders and significantly slowed post-crash response efforts. Improving safety, travel times and reliability, 

and WVCs through the Gap corridor represented the highest priorities. Public involvement during the PEL 

study included 8 public meetings, 3 telephone town halls, and numerous listening sessions. Public involvement 

during the EA comprised 2 public meetings and 2 public hearings, although Steering Committee meetings 

during the EA process were open to the public as well. Over the course of the combined PEL study and EA 

phase of the project, CDOT conducted 22 Technical Working Group meetings and 16 Regulatory Agency 

Group meetings along with numerous Steering Committee meetings. 

Safety in Purpose and Need

Safety was paramount in the project purpose statement, as well as in the documentation of project needs 

within the EA. CDOT clearly articulated and discussed safety needs as a result of preliminary safety analyses
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through the PEL study, which indicated that the 18-mile Gap corridor had the potential for safety 

improvements, particularly in the areas of roadway safety, WVCs, and incident response.

Safety in Mitigation

CDOT monitored environmental commitments and mitigation measures using the agency’s mitigation 

commitments tracking summary sheet. The most important safety-related commitment involved the proposed 

wildlife underpasses and other wildlife safety improvements. 
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References & Additional Information

» Attardo, C. (2020). I-25 South Gap Project: Monument-Castle Rock, TRB Webinar #3 [PowerPoint Slides].

» Colorado Department of Transportation. (2018). Environmental Assessment – I-25 South Gap: Monument 

to Castle Rock. Available online: https://www.codot.gov/projects/i-25-south-monument-castle-rockea/environmental-assessment. 

» Colorado Department of Transportation. (2019). I-25 Planning and Environmental Linkages Study: 

Colorado Springs Denver South Connection. Available online: 

https://www.codot.gov/projects/I25COSDEN. 

» Colorado Department of Transportation. (2022). Planning and Environmental Linkages (PEL) Handbook - 

Version 3. Available online: https://www.codot.gov/programs/environmental/planning-env-linkprogram/2022pelhandbook_final_061322.pdf.

» Federal Highway Administration. (2011a). HSIP Noteworthy Practice Series, HSIP Project Identification, 

Report No. FHWA-SA-11-02. Available online: 

https://rspcb.safety.fhwa.dot.gov/noteworthy/html/projident_co.aspx. 

» Federal Highway Administration (2011b). Integrating Road Safety into NEPA Analysis: A Primer for Safety 

and Environmental Professionals, Report No. FHWA-SA-11-36, Washington D.C. Available online:

https://rosap.ntl.bts.gov/view/dot/42549. 

» Federal Highway Administration. (2016a). Colorado’s Implementation of Level of Service of Safety, Strategies 

for Using GIS to Advance Highway Safety. FHWA-SA-16-027 Available online:

https://rosap.ntl.bts.gov/view/dot/50435. 

» Federal Highway Administration. (2016b). Use of Narrow Lanes and Narrow Shoulders on Freeways: A 

Primer on Experiences, Current Practice, and Implementation Considerations, Report No. FHWA-HOP-16-

060. Available online: https://rosap.ntl.bts.gov/view/dot/42439. 

» Phone Interview on January 19, 2022, with Chuck Attardo (CDOT). 

» Phone Interview on April 18, 2022, with Chuck Attardo (CDOT).

Project Contacts

Chuck Attardo

Colorado Department of Transportation

(303) 365-7211 

chuck.attardo@state.co.us

Brian Dobling

Federal Highway Administration – Colorado Division

(720) 963-3032 

brian.dobling@dot.gov