TECHBRIEF

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

www.fhwa.dot.gov/research

Safety Evaluation of Corner 

Clearance at Signalized 

Intersections

FHWA Publication No.: FHWA-HRT-17-085

FHWA Contact: Roya Amjadi, HRDS-20, 

(202) 493-3383, roya.amjadi@dot.gov

This document is a technical summary of the Federal 

Highway Administration report Safety Evaluation of 

Corner Clearance at Signalized Intersections (FHWAHRT-17-084).

Objective

The Federal Highway Administration (FHWA) established the Development of Crash Modification Factors 

(DCMF) program in 2012 to address highway safety 

research needs for evaluating new and innovative 

safety strategies (improvements) by developing reliable quantitative estimates of their effectiveness in 

reducing crashes. The ultimate goal of the DCMF 

program is to save lives by identifying new safety 

strategies that effectively reduce crashes and promote those strategies for nationwide implementation 

by providing measures of their safety effectiveness 

and benefit–cost (B/C) ratios through research. State 

transportation departments and other transportation 

agencies need to have objective measures for safety 

effectiveness and B/C ratios before investing in broad 

applications of new strategies for safety improvements. Forty State transportation departments provided technical feedback on safety improvements 

to the DCMF program and implemented new safety 

improvements to facilitate evaluations. These States 
2

are members of the Evaluation of LowCost Safety Improvements Pooled Fund 

Study, which functions under the DCMF 

program. This study has evaluated 

corner clearance at signalized intersections. The intent of this strategy is to 

reduce the frequency of angle crashes at 

intersections by increasing the distance 

between driveways and intersections.

Several research studies explored 

management techniques and the effects 

of access points on safety at a corridor 

level. However, little quantitative 

information is available for the safety 

effects of driveways located near the 

corners of a signalized intersection 

and the effects of access management 

strategies on intersection crashes. This 

study investigates the safety effects of 

corner clearance on the mainline at fourleg, signalized intersections.

Introduction

Corner clearance is defined as the distance between an intersection and the 

nearest driveway or access point along 

the approach. Adequate corner clearance is an important factor in the safety 

and operations at intersections. The 

American Association of State Highway 

and Transportation Officials (AASHTO) 

publication A Policy on Geometric 

Design of Highways and Streets (also 

known as “The Green Book”) notes that 

driveways should not be located within 

the functional area of an at-grade intersection or in the influence area of an 

adjacent driveway.(1) However, the presence of conflicting driveways within the 

functional area is often unavoidable, 

especially in urban environments.

While inadequate corner clearance is a 

concern for all types of intersections, 

signalized intersections develop recurring queues within the functional area 

of the intersection that can lead to conflicts with vehicles turning in and out 

of driveways. Approaches to signalized 

intersections also have more lanes on 

average than other types of at-grade 

intersections, which can cause difficulties for drivers leaving driveways to 

weave and maneuver into their desired 

lanes.

Figure 1 shows a photo of a signalized 

intersection with limited corner clearance. Figure 2 shows a general layout of 

the study site.

The research team conducted a literature review that concentrated on the 

safety effects of corner clearance distance as well as access spacing and 

various property access management 

strategies. Most evaluations to date 

have focused on corridor safety effects 

rather than intersection safety.
3

© VHB

Figure 1. Photo. Signalized intersection with limited corner clearance.

Figure 2. Schematic. General layout of study site.
4

Kwigizile et al. examined changes in 

the number of crashes at urban signalized intersections as a result of corner 

clearance and other variables.(2) A zeroinflated negative binomial model was 

selected from four model forms as the 

best model for determining the safety 

effects of the treatment. The authors 

modeled corner clearance as the number of corner clearances (i.e., number of 

access points) and the average corner 

clearance in feet, with a maximum of 

250 ft.

The results indicated that increased corner clearance and fewer access points 

yielded fewer crashes. Commercial 

driveways with limited corner clearance 

led to higher crash rates than residential accesses. Signals with higher minor 

road volumes had a higher number of 

crashes. Crashes generally increased 

with the addition of left-turn lanes and 

through lanes, with through lanes leading to a greater increase.

Methodology

For the purpose of this project, the 

research team examined the safety 

effects of corner clearance at signalized 

intersections in the State of California 

and the City of Charlotte, North 

Carolina. The objective was to estimate 

the safety effectiveness of this strategy 

as measured by crash frequency. The 

research team included the following 

crash types:

• Total: all crashes within 250 ft of 

intersection (all types and severity 

levels combined).

• Fatal and Injury: all injury crashes 

within 250 ft of intersection (fatal 

injury, incapacitating injury, nonincapacitating injury, and possible 

injury).

• Rear-end: all crashes coded as “rearend” within 250 ft of intersection.

• Sideswipe: all crashes coded 

as “sideswipe” within 250 ft of 

intersection.

• Right-angle: all crashes coded as 

“right-angle” within 250 ft of 

intersection.

• Turning: all crashes coded as “rightturn” or “left-turn” within 250 ft of 

intersection.

• Nighttime crashes: all crashes with 

light condition coded as “night,” 

“dawn,” or “dusk” within 250 ft of 

intersection.

A further objective of the project was 

to conduct a disaggregate analysis to 

investigate whether the safety effects 

vary by factors such as the level of traffic on the mainline and cross streets, 

number of lanes, posted speed limit, 
5

driveway density, and the presence of 

exclusive right- and left-turn lanes.

The evaluation of overall effectiveness 

included the consideration of the installation costs and crash savings in terms 

of the B/C ratio.

The research team used a crosssectional study design for the evaluation. At the most basic level, the safety 

effect is estimated by taking the ratio 

of the average crash frequency for two 

groups, one with the treatment and the 

other without the treatment. The two 

groups of sites should be similar in all 

regards except for the presence of the 

treatment. This is difficult to accomplish 

in practice. The research team adopted 

the propensity score matching technique to match sites with and without 

the treatment while using multiple variable regression modeling to control for 

other characteristics that vary among 

sites.

The research team employed multiple variable regression to develop the 

statistical relationships between the 

dependent variables and a set of predictor variables. In this case, crash frequency was the dependent variable, and 

the team considered several predictor 

variables, including number of corners 

with limited clearances, traffic volumes, 

and other intersection and roadway 

characteristics. The research team considered regression coefficients during 

the modeling process for each predictor 

variable. The coefficients represent the 

expected change in crash frequency due 

to a unit change in the predictor variable 

with all else being equal.

The research team applied generalized 

linear modeling techniques to develop 

the crash prediction models and specified a log-linear relationship using a 

negative binomial error structure. The 

negative binomial error structure has 

advantages over the Poisson distribution in that it allows for overdispersion 

of the variance that is often present in 

crash data.

The full report includes a detailed 

explanation of the methodology, including a description of how the estimate 

of safety effects for target crashes was 

calculated.

Results

The results are presented in two parts. 

The first part contains aggregate results, 

and the second part is based on a disaggregate analysis that sought to identify 

those conditions under which the strategy is most effective.

Aggregate Analysis

The study presented aggregate results 

separately for the number of approach 
6

corners (i.e., one or two) and number of receiving corners (i.e., one or 

two) with driveways within 50 ft of the 

intersection compared to no driveways within 50 ft of the intersection 

on the approach or receiving corners, 

respectively. Table 1 and table 2 provide 

the crash modification factor (CMF) 

estimates for all crash types.

The CMFs associated with the receiving 

corners were consistent with expectation, indicating crash increases for limited corner clearance (i.e., driveways 

within 50 ft of one or both receiving corners) compared to no driveways within 

50 ft of both receiving corners. However, 

the results associated with the approach 

corners appear counterintuitive, indicating crash decreases for limited 

Crash Type CMF for 1 Receiving Corner With 

Driveway(s) Within 50 ft (S.E)

CMF for 2 Receiving Corners With 

Driveway(s) Within 50 ft (S.E) 

Total crashes 1.33** (0.11) 1.76** (0.30)

Fatal and injury crashes 1.29** (0.11) 1.68** (0.29)

Rear-end crashes 1.36** (0.14) 1.86** (0.38)

Sideswipe crashes 1.31** (0.14) 1.71* (0.38)

Right-angle crashes 1.42** (0.20) 2.02* (0.56)

Turning crashes 1.22 (0.15) 1.49 (0.36)

Nighttime crashes 1.29** (0.13) 1.67* (0.35)

*Indicates statistically significant results at the 90-percent confidence level.

**Indicates statistically significant results at the 95-percent confidence level.

Crash Type CMF for 1 Approach Corner With 

Driveway(s) Within 50 ft (S.E)

CMF for 2 Approach Corners With 

Driveway(s) Within 50 ft (S.E) 

Total crashes 0.82** (0.08) 0.67** (0.13)

Fatal and injury crashes 0.79** (0.08) 0.62** (0.13)

Rear-end crashes 0.79** (0.09) 0.63** (0.15)

Sideswipe crashes 0.83 (0.12) 0.69 (0.19)

Right-angle crashes 1.03 (0.16) 1.06 (0.34)

Turning crashes 1.00 (0.15) 1.01 (0.30)

Nighttime crashes 0.94 (0.12) 0.87 (0.23)

Table 1. Estimated CMFs for limited approach corner clearance.

**Indicates statistically significant results at the 95-percent confidence level.

Table 2. Estimated CMFs for limited receiving corner clearance.
7

corner clearance (i.e., driveways within 

50 ft of one or both approach corners) 

compared to no driveways within 50 ft 

of both approach corners. It is important 

to note that the CMFs from table 1 and 

table 2 are independent, and the results 

do not suggest crash migration from 

the receiving corners to the approach 

corners. Readers can refer to the full 

project report for a more detailed 

discussion of the results and a few 

possible explanations for the results 

that are counter to the general hypothesis of the study.

Disaggregate Analysis

The objective of the disaggregate 

analysis was to identify those 

conditions under which the strategy 

is most effective. The research team 

considered several variables in the 

disaggregate analysis, including major 

and minor road traffic volumes, number 

of lanes on the major and minor road, 

posted speed limit, driveway density, 

and presence of left- and right-turn 

lanes. The multiple variable regression 

models included interaction terms to 

investigate the potential differential 

effects of corner clearance with respect 

to the interacted variable. For example, 

the interaction term for major road 

traffic volume and number of major 

road approaches with driveways within 

50 ft is the product of the two variables. 

A statistically significant interaction 

term would indicate an apparent 

differential effect of corner clearance 

across different traffic volumes or the 

other variables of interest.

The results indicated that none of the 

interaction terms were statistically 

significant at even the 80-percent 

confidence level. While these results 

indicate no differential effect of corner 

clearance, it is possible that the sample 

size is too small and/or there was little 

variation in the variables of interest to 

detect differential effects at the desired 

level of confidence.

Economic Analysis

For the economic analysis, the research 

team assumed increasing corner 

clearance involved the removal of 

driveways with corner clearance of 50 ft 

or less by installing concrete curbing 

and sidewalk in place of the mainline 

access for a commercial property. 

The intent was to shift traffic to an 

existing access on the cross street or 

further downstream (corner clearance 

more than 50 ft) on the mainline. The 

cost did not include the construction 

of a new access point, which, if 

necessary, would drastically increase 

the estimated cost of the treatment. 

The research team assumed another 

mainline or cross street access 

could continue to provide access to the 

property. Based on cost information for 

concrete sidewalks with curb and gutter 
8

from the National Cooperative Highway 

Research Program (NCHRP) Report 

500 Volume 10: A Guide for Reducing 

Collisions Involving Pedestrians, curbing costs an average of $15 per linear ft 

and walkways cost $11 per square ft.(3)

Assuming a sidewalk width of 6 ft, the 

average installation cost is $81 per linear 

ft of curb and sidewalk. Although most 

access points are narrower, the analysis 

used a conservative assumption of 

100 ft of curb and sidewalk to connect 

walkways on either side of an existing 

driveway. Given these assumptions, the 

construction cost for removing access 

points is approximately $8,100 per 

access point per corner. The project 

team assumed the construction cost per 

corner was the same regardless of the 

number of corners treated.

The FHWA Office of Safety Research and 

Development suggested using the Office 

of Management and Budget Circular A-4 

as a resource for the real discount rate 

of seven percent to calculate the present 

value benefits and costs of the treatment during the service life.(4) With this 

information, the analysis used a Capital 

Recovery Factor of 7.02.

The research team used FHWA’s mean 

comprehensive crash costs by 

crash geometry as a basis for the 

benefit calculations.(5) The mean comprehensive crash cost for a fatal and 

injury crash was $158,177 in 2001 

dollars. The cost for a property damage only crash was $7,428 in 2001 U.S. 

dollars (USD). The team weighted 

these values using the distribution of 

crash severities across study sites (i.e., 

approximately 43 percent fatal and 

injury crashes) to determine the mean 

comprehensive cost of a total crash as 

$71,553 in 2001 USD. At the time of 

analysis, the team updated this value 

to 2016 USD by applying the ratio of 

the U.S. Department of Transportation 

(USDOT) 2016 value of a statistical life 

of $9.6 million to the 2001 value of 

$3.8 million, yielding an aggregate 2016 

cost of $176,998 for a total crash.(5,6)

To determine the safety benefits of 

increasing corner clearance, the project 

team analyzed the safety effects of 

removing access points with less 

than 50 ft corner clearance on one or 

both mainline receiving corners of an 

average signalized intersection that 

had two receiving corners with limited 

corner clearance. The data summary 

in the full report shows an average 

crash frequency of 4.36 crashes per 

site per year at four-legged, signalized 

intersections with no limited corner 

clearance on receiving corners. The 

research team multiplied the average 

crash frequency by the total crash CMFs 

of 1.33 and 1.76 to estimate the crash 

frequency at sites with limited clearance 

on one (5.80 crashes per site per year) 

and two receiving corners (7.67 crashes 
9

per site per year). The research team 

used the differences in crash frequency 

between sites with two and one limited 

clearance corners (1.87 crashes per 

site per year) and two and zero limited 

clearance corners (3.31 crashes per site 

per year) as the average reduction of 

total crashes in each scenario.

The research team calculated the 

annual economic benefits by multiplying the total crash reduction per site per 

year by the average cost of a total crash 

and then annualizing the result during 

the service life. USDOT recommended 

conducting a sensitivity analysis by 

assuming values of a statistical life of 

0.55 and 1.38 times the 2016 value as 

lower and upper bounds.(6) Researchers 

can apply these factors directly to the 

estimated B/C ratios. Table 3 presents 

the results.

These results suggested that removing 

access on mainline receiving corners to 

improve corner clearance, with reasonable assumptions on cost, service life, 

and the value of a statistical life, can be 

cost effective for reducing crashes at 

signalized intersections.

Practitioners should conduct their 

own economic analysis for the site at 

which they are considering access point 

relocation or removal, with crash and 

project cost information specific to that 

site. This analysis demonstrates the 

process that practitioners could use for 

economic analysis.

Summary and Conclusions

This study’s objective was to undertake 

a rigorous cross-sectional evaluation of 

the safety effects, as measured by crash 

frequency, of mainline corner clearance 

at four-legged, signalized intersections. 

The study compared signalized 

intersections with various corner 

clearance using data from California 

and Charlotte, North Carolina, to 

examine the effects on specific crash 

types: total, fatal and injury, rearend, sideswipe, right-angle, turning, 

and nighttime crashes. The study 

did not investigate the effects of 

corner clearance on the cross-street 

approaches or intersections with three 

legs or more than four legs.

The estimated CMFs indicated that 

more limited clearance on receiving 

Number of Access Points With Limited 

Corner Clearance Removed Lower B/C Average B/C Upper B/C

1 161.6 293.9 405.5

2 285.7 519.4 716.7

Table 3. B/C ratios for removing receiving corner access points from a site with limited clearance on two receiving 

corners.
10

corners (i.e., driveway(s) within 50 ft 

of the signalized intersection) was 

associated with increases for all crash 

types, based on the data included in 

this analysis. These increases were 

statistically significant at the 90-percent 

level or greater for total, fatal and 

injury, rear-end, sideswipe, right-angle, 

and nighttime crashes. The total crash 

CMFs are 1.33 and 1.76 (SE = 0.11 and 

0.30) for one and two receiving corners, 

respectively. Similarly, the CMFs for 

fatal and injury crashes are 1.29 and 1.68 

(SE = 0.11 and 0.29). The CMFs for rearend crashes are 1.36 and 1.86 (SE = 0.14 

and 0.38). The CMFs for sideswipe 

crashes are 1.31 and 1.71 (SE = 0.14 and 

0.38). The CMFs for right-angle crashes 

are 1.42 and 2.02 (SE = 0.20 and 0.56). 

The CMFs for turning crashes are 1.22 

and 1.49 (SE = 0.15 and 0.36). The CMFs 

for nighttime crashes are 1.29 and 1.67 

(SE = 0.13 and 0.35). Only the results 

for turning crashes were not statistically 

significant at the 90-percent level.

For limited corner clearance on the 

approach corners, the results indicated 

statistically significant reductions in 

total, fatal and injury, and rear-end 

crashes. The results also indicated 

reductions in sideswipe and nighttime 

crashes, and increases in right-angle 

and turning crashes, but none of these 

results were statistically significant at 

the 90-percent level. In other words, each 

additional mainline approach corner 

with at least one driveway within 50 ft 

of the corner is statistically associated 

with decreases in these crash types. 

Although unintuitive, this may be the 

result of localized congestion on the 

approach corners of an intersection. 

The total crash CMFs for one and two 

approach corners are 0.82 and 0.67 

(SE = 0.08 and 0.13), respectively. 

Similarly, the CMFs for fatal and injury 

are 0.79 and 0.62 (SE = 0.08 and 0.13). 

The CMFs for rear-end crashes are 

0.79 and 0.63 (SE = 0.09 and 0.15). The 

estimated CMFs for sideswipe, right 

angle, turning, and nighttime crashes 

indicate a mix of no changes, a slight 

increase, or a slight decrease in crashes 

associated with limited clearance on the 

approach corners and none of these 

results are statistically significant.

The disaggregate analysis did not reveal 

any differential effect of corner clearance at the 80-percent confidence level.

The economic analysis, based on total 

crashes and assuming a 10-year service 

life, resulted in an average B/C ratio of 

at least 294 to 1 for most intersections 

when removing or relocating access at 

one or more mainline receiving corners 

with limited corner clearance. With 

the USDOT recommended sensitivity 

analysis, these values could range 
11

from 162 to 1 up to 405 to 1. While 

this research suggested the presence 

of driveways on mainline approach 

corners does not increase total, fatal 

and injury, rear-end, and sideswipe 

crashes, more research is required 

before agencies may consider this as a 

strategy for reducing crashes.

These results suggest that removing or 

relocating driveways on the mainline 

receiving corners can be highly cost 

effective in reducing crashes at fourlegged, signalized intersections.

References

1. American Association of State 

Highway and Transportation Officials. 

(2011). A Policy on Geometric Design 

of Highways and Streets, 6th ed., 

AASHTO, Washington, DC.

2. Kwigizile, V., Mulokozi, E., Xu, 

X., Teng, H., and Ma, C. (2014). 

“Investigation of the Impact of Corner 

Clearance on Urban Intersection 

Crash Occurrence,” Journal of 

Transportation and Statistics, 10(1), 

Bureau of Transportation Statistics, 

U.S. Department of Transportation, 

Washington, DC.

3. Zegeer, C., Stutts, J., Huang, H., 

Cynecki, M., Van Houten, R., Alberson, 

B., Pfefer, R., Neuman, T., Slack, K., 

and Hardy, K. (2004). Guidance for 

the Implementation of AASHTO 

Strategic Highway Safety Plan, 

Volume 10: A Guide for Reducing 

Collisions Involving Pedestrians, 

National Cooperative Highway 

Research Program Report 500, 

Transportation Research Board of 

the National Academies, Washington, DC.

4. Office of Management and Budget. 

(2003). Circular A-4: Regulatory 

Analysis, Office of Management and 

Budget, Washington, DC.

5. Council, F., Zaloshnja, E., Miller, T., 

and Persaud, B. (2005). Crash Cost 

Estimates by Maximum PoliceReported Injury Severity within 

Selected Crash Geometries, FHWAHRT05-051, Federal Highway 

Administration, Washington, DC.

6. U.S. Department of Transportation. 

(2016). Guidance on Treatment 

of the Economic Value of a 

Statistical Life (VSL) in U.S. 

Department of Transportation 

Analyses—2016 Adjustment, 

Memorandum, obtained from: 

https://cms.dot.gov/sites/dot.gov/

files/docs/2016%20Revised%20

Value%20of%20a%20Statistical%20

Life%20Guidance.pdf.
12

FEBRUARY 2018 FHWA-HRT-17-085

HRDI-20/02-18(200)E

Researchers—Thanh Le, Frank Gross, Tim Harmon, and Kimberly Eccles.

Distribution—This TechBrief is being distributed according to a standard 

distribution. Direct distribution is being made to the Divisions and Resource Center.

Availability—This TechBrief may be obtained from the FHWA Product Distribution 

Center by e-mail to report.center@dot.gov, fax to (814) 239-2156, phone to (814) 239-

1160, or online at http://www.fhwa.dot.gov/research.

Key Words—Corner clearance, Signal, Intersection, Low-cost, Safety improvements, 

Safety evaluations, Empirical Bayesian.

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