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Publication Number:  FHWA-HRT-14-057    Date:  February 2018
Publication Number: FHWA-HRT-14-057
Date: February 2018

 

Safety Evaluation of Access Management Policies and Techniques

CHAPTER 1. INTRODUCTION

 

STUDY BACKGROUND

Access management (AM) is the process that provides (or manages) access to land development while preserving the flow of traffic on the surrounding road network in terms of safety, capacity, and speed. AM provides important benefits to the transportation system. These benefits have been increasingly recognized at all levels of government, and a growing number of States, cities, counties, and planning regions are managing access by requiring driveway permit applications and establishing where new access should be allowed. These agencies are also closing, consolidating, or improving driveways, median openings, and intersections as part of their AM implementation strategy. However, these decisions are often challenged for various reasons.

There is a need for additional information, which would help agencies make decisions related to AM and better explain the safety and operational benefits of their policies and practices. Previous studies and empirical evidence have shown positive operational and safety benefits associated with good AM practices. While the operational effects of AM have been investigated quantitatively through different modeling and analysis approaches, there have been few scientifically rigorous evaluations to quantify the safety effectiveness, particularly for corridor AM. The Federal Highway Administration (FHWA) initiated this study to help fill some of the research gaps—namely, to quantify the safety impacts of corridor AM decisions.

The study team solicited input from a panel of State and local representatives to identify AM principles and design factors that should be included in a corridor-level model. Based on input from the panel and availability of data, the study team prioritized the principles as shown in table 1.

Table 1. Prioritization of AM policies and techniques.

AM Strategy Applicable AM Principles Priority
Establish unsignalized access spacing criteria. Limit the number of conflict points.
Separate conflict areas.		 1
Establish signal spacing criteria. Locate signals to favor through movements.
Limit the number of conflict points.
Separate conflict areas.		 1
Establish spacing criteria for interchange crossroads. Limit the number of conflict points.
Separate conflict areas.		 1
Establish spacing criteria for median openings/crossovers. Limit the number of conflict points.
Separate conflict areas.		 1
Establish corner clearance criteria. Preserve the functional area of intersections.
Separate conflict areas.		 1
Provide median and accommodate left turns and U-turns. Limit the number of conflict points.
Separate conflict areas.
Manage left-turn movements.		 1
Provide left-turn lane. Remove turning vehicles from through-traffic lanes. 1
Close or modify median opening and accommodate left turns and U-turns. Limit the number of conflict points.
Separate conflict areas.
Manage left-turn movements.		 1
Provide TWLTL. Remove turning vehicles from through-traffic lanes. 2
Provide right-turn lane. Remove turning vehicles from through-traffic lanes. 2
Provide frontage/backage road. Limit the number of conflict points.
Remove turning vehicles from through-traffic lanes.		 2
Provide internal cross connectivity. Limit the number of conflict points.
Remove turning vehicles from through-traffic lanes.		 2
TWLTL = two-way left-turn lane; 1 = highest priority; 2 = secondary priority.

 

OVERVIEW OF STRATEGIES

The following AM strategies/policies are considered in this research project and discussed in the following sections:

Table 2 illustrates how these strategies/policies relate to achieving the safety objectives of basic AM principles.

Table 2. Strategies/policies in relation to AM safety principles (adapted from V.G. Stover, 2007).(1)

AM Principle AM Strategy/Policy Limit Conflicts Separate Conflicts Reduce Conflicts
Access spacing Unsignalized access spacing — X —
Access spacing Traffic signal spacing — X —
Access spacing Interchange crossroad spacing — X —
Access spacing Corner clearance — X —
Roadway cross section Median type: TWLTL — — X
Roadway cross section Median type: nontraversable median X — —
Roadway cross section Median type: Replace TWLTL with nontraversable median X X —
Roadway cross section Directional median opening X — —
Roadway cross section Median opening spacing — X —
Property access Frontage/backage roads X X —
Property access Internal cross connectivity X X —
TWLTL = two-way left-turn lane; X = the strategy/policy can achieve the specified safety objective.
—The strategy/policy does not accomplish the specified safety objective.

 

Access Spacing

Unsignalized Access Spacing (Intersections and Driveways)

Access points, commonly referred to as “driveways” or “street intersections,” introduce conflicts and friction into the flow of traffic along a roadway. Vehicles entering and leaving the roadway often slow the movement of through traffic, and the difference in speeds between through-traffic and turning-traffic increases the potential for crashes. American Association of State Highway Transportation Officials’ (AASHTO) A Policy on Geometric Design of Highways and Streets (i.e., the “Green Book”) indicates that the number of crashes is disproportionately higher at driveways than at other intersections. Therefore, driveway design and location merit special consideration (pp. 729–731).(2)

Where an access point is needed, its location should be selected to minimize its adverse effects on roadway safety and traffic flow. Increasing the spacing between access points through proper planning of future access and closing or consolidating existing access improves traffic flow and safety along the roadway by achieving the following:

Figure 1 illustrates the spacing distance between two adjacent unsignalized driveways, where the distance is measured from the nearest edges of each driveway. Some agencies choose to measure the spacing distance from the centerlines of the adjacent driveways.

Figure 1. Illustration. Unsignalized driveway spacing. This figure shows a two-lane, undivided road aligned horizontally on the page. Two adjacent stop-controlled driveways are located along the roadway, and both of the driveways are located on the south side. An arrow labeled “driveway spacing” spans between the driveways, indicating that the driveway spacing is measured from the right edge of the driveway on the left to the left edge of the driveway on the right.
Source: FHWA.

Figure 1. Illustration. Unsignalized driveway spacing.

 

Traffic Signal Spacing

Establishing traffic signal spacing criteria for arterial roadways is one of the most important and basic AM techniques. These criteria apply to both signalized driveways and signalized public roadway intersections.

The proper spacing of traffic signals, in terms of frequency and uniformity, is one of the most important and basic AM techniques because of the effects traffic signals have on arterial safety and traffic flow. Frequency refers to the number of traffic signals for a given length of roadway and is sometimes referred to as “signal density.” It is typically expressed as the number of signals per mile. Uniformity refers to the variation in the distances between individual traffic signals along a given length of roadway. It is desirable to minimize this variation and to space traffic signals at uniform distances as shown in figure 2.

Figure 2. Illustration. Comparison of uniform and nonuniform signal spacing (figure 5 in Gluck, Levinson, and Stover, 1999). This figure depicts two types of traffic signal spacing. The diagram on the top illustrates the example of uniform traffic signal spacing. Seven traffic signals are positioned 0.5 mile apart along a 3-mile roadway. The diagram on the bottom shows the example of non-uniform traffic signals spacing. Five traffic signals are spaced at non-uniform distances along the first two miles of roadway, and two signals are located within the third mile of roadway. In both cases, the signal density is 2.0 signals per mile and the signal frequency is 7.0 signals, but the uniformity is different.
©Transportation Research Board.

Figure 2. Illustration. Comparison of uniform and nonuniform signal spacing (figure 5 in Gluck, Levinson, and Stover, 1999).(3)

 

Closely spaced or improperly spaced traffic signals can result in increased crash rates, frequent stops, unnecessary delays for motorists and pedestrians, increased fuel consumption, and excessive vehicular emissions.

For example, if a 2-mi segment of roadway would require four traffic signals (i.e., a signal density of two signals/mile), it is generally more desirable to space the signals at a uniform distance along the roadway (e.g., every ½ mi), rather than space them irregularly (e.g., 1 mi, ¼ mi, ½ mi, and ¼ mi). Properly spaced traffic signals allow for the efficient progression of motor vehicle and pedestrian traffic as well as provide an agency with greater flexibility in developing signal timing plans to reduce traffic conflicts.

Interchange Crossroad Spacing

Freeway interchanges provide the means of moving traffic between freeways and intersecting crossroads. Although direct property access is prohibited on the freeway itself, safety and operational problems can arise when driveways and intersections along the crossroad are spaced too close to the interchange ramp termini. Heavy weaving volumes, complex traffic signal operations, frequent crashes, and recurrent congestion could result. In addition, driveways and median breaks that are provided for direct access to properties along the crossroad compound these problems.

Managing access on crossroads in the vicinity of interchanges protects the longevity of both the interchange and the intersecting crossroad by reducing crash rates, minimizing congestion, and simplifying driving tasks. Improperly managing access on the crossroad near the interchange may cause congestion and potential crashes, thereby shortening the life cycle of the interchange. In addition, it may cause significant impairment of crossroad and freeway mainline safety and operations. For these reasons, AM should be applied to interchange crossroads such that access points, including both driveways and intersections, are sufficiently separated from freeway interchange ramp termini.

Corner Clearance

Protecting the functional integrity of intersections is extremely important from the safety and operations perspectives. One strategy to help accomplish this is to locate driveways outside the functional area of an intersection. As shown in figure 3, the intersection functional area extends beyond the physical intersection limits to include the upstream approaches, where deceleration, maneuvering, and queuing take place, as well as the downstream departure area beyond the intersection, where driveways could introduce conflicts and generate queues backing up through the intersection. As noted in AASHTO’s A Policy on Geometric Design of Highways and Streets, driveways should not be located within the functional area of an intersection or in the influence area of an adjacent driveway.(2)

Figure 3. Illustration. Intersection physical area versus functional area (adapted from Transportation Research Circular 456, figure 4). This figure includes two diagrams that illustrate intersection physical area and intersection functional area, respectively. A four-leg intersection is used in both diagrams to illustrate these concepts. In both cases, the north–south route is a two-lane undivided road, and the east–west route is a two-lane median-divided road. The diagram on the top illustrates the physical area of the intersection, which is the area surrounded by the stop bars of each approach. The physical area of the intersection is shaded for emphasis. The diagram on the bottom illustrates the functional area of the intersection, which is greater than the physical area of the intersection. Again, the functional area of the intersection is shaded for emphasis. An arrow labeled “Functional Area of Westbound Direction” indicates that the functional area for the westbound direction includes the upstream approaches, during which deceleration, maneuvering, and queuing take place, as well as the downstream departure area. The upstream and downstream areas of the other approaches are also shaded to indicate that the functional area includes the intersection approach and departure areas.
©Transportation Research Board.

Figure 3. Illustration. Intersection physical area versus functional area (adapted from Transportation Research Circular 456, figure 4).(4)

Roadway Cross Section

Median Type (Raised, TWLTL, Undivided)

Installations of nontraversable (i.e., raised) medians with provisions for median openings to accommodate left turns and U-turns have proven to be among the most effective techniques for reducing conflicts and improving traffic operations along roadways. The installation of a nontraversable median reduces the number of conflicts along a highway corridor by restricting driveways (not located at median openings) to right-in/right-out movements and directing left-turn and U-turn movements to designated median openings as shown in figure 4.

Figure 4. Illustration. Allowable traffic movements before and after raised median installation (figure 30 in Gluck, Levinson, and Stover, 1999). This figure uses two diagrams to compare the traffic movements allowed with and without a raised median. Each diagram shows a four-lane roadway (oriented in the east–west direction), with two four-leg intersections: one on each end of the corridor, and one three-leg intersection in the middle. The diagram on the top depicts the undivided scenario, which is the period before installation of a raised median. The diagram on the top illustrates and notes that all turns are permitted and there are many conflict points. There are 40 conflict points at each four-leg intersection and 11 conflict points at the three-leg intersection. The diagram on the bottom illustrates and notes that the conflict points are reduced at all intersections with the installation of a continuous median. Specifically, the four-legged intersections now have 4 potential conflict points, 2 for each leg, and the three-legged intersection has 2 potential conflict points. The bottom diagram also notes that there is increased traffic at the first median opening.
© Transportation Research Board.

Figure 4. Illustration. Allowable traffic movements before and after raised median installation (figure 30 in Gluck, Levinson, and Stover, 1999).(3)

 

Allowing unrestricted left-turn movements to and from all access driveways increases the number of vehicular conflict points with other vehicles, pedestrians, and bicyclists. Left-turning vehicles have been shown to account for nearly three-quarters (74 percent) of all access-related crashes.

Nontraversable medians with designated median openings to allow for left-turn and U-turn movements offer the following advantages over the other types of roadway cross sections:

Median Opening Spacing

The Florida Department of Transportation’s Median Handbook (Interim Version) indicates that “restrictive medians and well-designed median openings are known to be some of the most important features in a safe and efficient highway system.”(5) A median opening is an opening in a nontraversable median that provides for crossing and turning traffic. A “full” median opening allows all turning movements, whereas a “partial” median opening allows only specific movements and physically prohibits all other movements. To realize the safety benefits, median openings should not encroach on the functional area of another median opening or intersection (see figure 3 for an illustration of a functional area).

Property Access

In addition to the location and design of access points for a specific property, there are also strategies to provide reasonable access for a particular property, or properties, such that the resulting access configuration provides for safer and more efficient traffic operations. The following two strategies of this type were included in this research project and are described below:

A third strategy, providing access via secondary roadways (i.e., a roadway that has a lower access classification than the intersecting primary roadway), was not included because of the implications it has for traffic circulation patterns on the surrounding roadway network.

Frontage/Backage Roads

A frontage road is an access roadway that is generally aligned parallel to a main roadway and is located between the right-of-way of the main roadway and the front building setback line. Frontage roads are used as an AM technique to provide direct access to properties and separate through traffic from local access-related traffic. This reduces the frequency and severity of conflicts along the main roadway as well as traffic delays. In addition, the resulting increase in spacing between intersections along the main roadway facilitates the design of auxiliary lanes for deceleration and acceleration, further improving traffic safety and operations. A “backage” road—also called a “reverse frontage road” or “reverse access”—serves a similar purpose but is located behind the properties that front the main roadway. Frontage and backage roads may be configured for one-way operation or two-way operation. Figure 5 illustrates one potential frontage road configuration.

Figure 5. Illustration. Potential frontage road configuration. This figure depicts one-way frontage roads between intersections. The diagram shows a four-lane divided “main roadway” traveling east–west with a four-legged signalized intersection located near the west end. The signalized intersection is at a collector road that travels north–south. There are properties along both sides of the main roadway. Each direction has access to a one-way frontage road serving the adjacent property where traffic enters at the upstream access to the frontage road and exits at the downstream opening, which is stop-controlled in this example. There are medians separating the frontage roads from the main roadway. There is driveway access to the properties along the frontage road.
Source: FHWA.

Figure 5. Illustration. Potential frontage road configuration.

 

Shared Driveways and Internal Cross-Connectivity

AM promotes the implementation of shared-access driveways and cross-access easements between (compatible) adjacent properties, where possible, which allow pedestrians and vehicles to circulate between properties without reentering the abutting roadway (see figure 6). The sharing of access driveways improves roadway safety and operations by reducing the number of conflict points and separating conflict points along these roadways. The longer spacing between access driveways also facilitates the provision of left-turn and right-turn lanes, eliminating conflicts between through and turning movements. In addition, smoother traffic flow on the abutting street helps to reduce the propensity for vehicular crashes and to increase egress capacity.

Figure 6. Illustration. Improved access configuration with cross connectivity. This figure depicts a shared driveway for the properties within a street block, which allows pedestrians and vehicles to circulate between properties without reentering the abutting roadway. In this example, there is a two-lane undivided arterial traveling east–west along the north side of the figure. There are two stop-controlled collector roads, one on each end of the corridor, that form three-legged intersections. Both collector roads are located to the south of the arterial. There are three properties in between the collectors that are adjacent to the main arterial. The two end properties have access to the respective collector roadways, and the middle parcel has access to the arterial roadway, indicated with a vertical, red, dashed arrow. The figure shows a horizontal, red, dashed line connecting all three parcels, which illustrates how internal connections among the three parcels can improve access.
Source: FHWA.

Figure 6. Illustration. Improved access configuration with cross connectivity.

 

LITERATURE REVIEW

This section relates the findings of previous research to the AM strategies that were considered for inclusion in the models to estimate the relationship between corridor AM and safety. Salient literature sources—including definitive AM-related research documents such as NCHRP Report 395, Capacity and Operational Effects of Midblock Left-Turn Lanes; NCHRP Report 420, Impacts of Access Management Techniques; and NCHRP Report 524, Safety of U-Turns at Unsignalized Median Openings—are reflected in this document to help establish what past research has shown to be the relationship between the selected AM strategies and safety.(6,3,7)

The following AM strategies/policies are being considered in this research project and discussed in the following sections:

Access Spacing

Unsignalized Access Spacing (Intersections and Driveways) and Corner Clearance

Gluck, Levinson, and Stover, in Impacts of Access Management Techniques (NCHRP Report 420), compiled numerous studies from the 1950s through the 1990s to identify the relationship between access frequency or density and crash rates.(3) Although the specific relationships vary, reflecting differences in road geometry, operating speeds, and intersection and driveway traffic volumes, the studies had a consistent finding that increasing the frequency/density of accesses translates into higher crash rates.

The research for NCHRP Report 420 included comprehensive safety analyses that were performed on crash information obtained for some 386 roadway segments in multiple States.(3) This produced a series of three graphs for quantifying the relationship between crash rates and signalized and unsignalized access densities. All three figures have been incorporated into the 2004 (and subsequent) edition(s) of AASHTO’s A Policy on Geometric Design of Highways and Streets.(2)

Figure 7 illustrates the increase in crash rates for each type of median treatment as the total access density increases. The addition of each driveway per mile in urban or suburban areas would increase the annual crash rate by 0.11 to 0.18 crashes per million vehicle-miles traveled (MVMT) on undivided highways and by 0.09 to 0.13 crashes per MVMT on highways with TWLTLs or nontraversable medians.

Figure 7. Graph. Relationship between total access points per mile and crash rate (figure 24 in Gluck, Levinson, and Stover, 1999). This figure depicts estimated crash rates by type of median in urban and suburban areas. The x-axis shows total signalized and unsignalized access points per mile (range of 0 to 80 in increments of 5), and the y-axis shows crash rate per million vehicle-miles (range of 0 to 12 in increments of 1). Three curves (from top to bottom) represent three median types: undivided, two-way left-turn lane, and non-traversable median. The curves indicate that crash rates are positively correlated with signalized and unsignalized access points per mile for the three median types. The undivided curve has the highest crash rates, beginning at about 3.9 at 10 access points per mile and increasing with access point density to about 10.5 at 70 access points per mile. Two-way left-turn lane has the second highest crash rates, beginning at about 3.5 at 10 access points per mile and increasing to about 9.2 at 70 access points. Non-traversable median has the lowest crash rates, beginning at about 2.9 at 10 access points per mile and increasing to about 8.2 at 70 access points per mile.
©Transportation Research Board.

Figure 7. Graph. Relationship between total access points per mile and crash rate (figure 24 in Gluck, Levinson, and Stover, 1999).(3)

 

Gluck and Levinson, in The Relationship Between Access Density and Accident Rates: Comparisons of NCHRP Report 420 and Minnesota Data (NCHRP Research Results Digest 247), supplemented the research presented in NCHRP Report 420 with respect to the relationship between access density and crash rates by using additional data from Minnesota to confirm these relationships.(8) The results showed crash rate patterns in Minnesota that were similar to those summarized in NCHRP Report 420. Specifically, both datasets exhibited an increase in crash rates as access density increased and as signal density increased and lower crash rates for nontraversable medians relative to undivided facilities. Note that Minnesota has a relatively low number of roadway miles with TWLTLs. These results supported the recommended safety indices presented in NCHRP Report 420.(3)

Huffman and Poplin, in The Relationship Between Intersection Density and Vehicular Crash Rate on the Kansas State Highway System, analyzed a variety of roadway classifications on the Kansas State Highway System, including two-lane undivided, four-lane undivided, and five-lane.(9) The two-lane and four-lane undivided classifications were further subdivided into urban and rural, whereas the five-lane classification was limited to urban highway segments. In all cases, crash rates were found to increase with increasing intersection density, indicating that intersection density has a direct bearing on the safety of the traveling public.

Eisele and Frawley, in Estimating the Safety and Operational Impact of Raised Medians and Driveway Density: Experiences From Texas and Oklahoma Case Studies, analyzed the relationship between access density and crash rates based on before–after studies of 11 corridors in Texas and Oklahoma that underwent access consolidation and/or median improvements.(10) The results of a linear-regression analysis indicated that increasing access density results in an increase in the crash rate, irrespective of the median type (undivided, TWLTL, or raised median). A regression line was plotted that yielded an R-squared value of 0.48. Although the regression line explained only about half of the variability in the data, the relationship between access density and crash rate was clearly found to be a positive correlation.

Signal Spacing

The research performed by Gluck and Levinson in The Relationship Between Access Density and Accident Rates: Comparisons of NCHRP Report 420 and Minnesota Data (NCHRP Research Results Digest 247) confirmed the relationship identified in NCHRP Report 420 between crash rates and signal density.(8) Both datasets exhibited an increase in crash rates as signal density increased. These results supported the recommended safety indices presented in NCHRP Report 420. Figure 8 illustrates the relationship developed between crash rate and access density, including the impacts of traffic signal density.

Figure 8. Graph. Relationship between access points per mile and crash rate (figure 26 in Gluck, Levinson, and Stover, 1999). This figure shows the relationship between crash rates and unsignalized access density for corridors with different signalized intersection densities in urban and suburban areas. The x-axis shows unsignalized access points per mile (range of 0 to 80 in increments of 5), and the y-axis shows crash rate per million vehicle-miles (range of 0 to 10 in increments of 1). Four curves (from top to bottom) represent four groups of access density: corridors with more than 6.0 signals per mile, corridors with 4.1 to 6.0 signals per mile, corridors with 2.1 to 4.0 signals per mile, and corridors with less than 2.1 signals per mile. The curves indicate that crash rates are positively correlated with unsignalized access points per mile for all four groups of signal density, but the slope of the curve is steeper for the first three groups and less steep for corridors with less than 2.1 signals per mile. Each curve begins at 10 access points per mile, and the values are: 6 crashes for corridors with more than 6.0 signals per mile, 4.8 crashes for corridors with 4.1 to 6.0 signals per mile, 3.9 crashes for corridors with 2.1 to 4.0 signals per mile, and 2.6 crashes for corridors with less than 2.1 signals per mile. Each curve ends at 70 access points per mile, and the values are: 9.5 crashes for corridors with more than 6.0 signals per mile, 8.8 crashes for corridors with 4.1 to 6.0 signals per mile, 8.1 crashes for corridors with 2.1 to 4.0 signals per mile, and 3.8 crashes for corridors with less than 2.1 signals per mile.
©Transportation Research Board.

Figure 8. Graph. Relationship between access points per mile and crash rate (figure 26 in Gluck, Levinson, and Stover, 1999).(3)

 

NCHRP Report 420 also presented information from Lee County, Florida, on the effects of traffic signal densities on crash rates from 1993. As shown in figure 9, doubling the signal density from two to four signals per mile increases the crash rate by approximately 2.5 times. As noted in NCHRP Report 420, the safety impacts of increased traffic signal spacing may be confounded, in part, by the traffic volumes on intersecting roadways and the common practice of using vehicle-miles of travel for comparing crash rates rather than the crashes per entering vehicles.

Figure 9. Graph. Relationship between signals per mile and crash rate (figure 6 in Gluck, Levinson, and Stover, 1999). This figure shows the relationship between signals per mile and crashes. The x-axis represents signals per mile (range of 0 to 5 in increments of 1) and the y-axis represents accidents per million vehicle-miles (range of 0 to 7 in increments of 1). The curve indicates that crashes rates are positively correlated with signals per mile. The curve begins at 0.5 crashes for 0 signals per mile and increases in an exponential shape to 7 crashes for 4.5 signals per mile. Several data points are scattered on the graph, and most fit closely to the curve with the exception of one high crash rate (2.5 crashes) at 0.5 signals per mile.
©Transportation Research Board.

Figure 9. Graph. Relationship between signals per mile and crash rate (figure 6 in Gluck, Levinson, and Stover, 1999).(3)

 

Schultz, Braley, and Boschert, in Correlating Access Management to Crash Rate, Severity, and Collision Type, applied stepwise linear regression analysis to identify correlations between AM techniques and crash patterns.(11) The results indicated that crash rates were correlated with signals per mile and that, on average, each signal corresponded to 0.92 crash per MVMT.

Interchange Crossroad Spacing

There has been limited research on the relationship between safety and the spacing of access points in the vicinity of an interchange. Rakha and the other researchers involved in the project that produced the Access Control Design on Highway Interchanges report developed a methodology to evaluate the safety impacts of different access spacing standards on crossroads at interchanges. The analysis results demonstrated the shortcomings of the 100-ft urban spacing guideline.(12)

The relationship between safety and the spacing of access points in the vicinity of an interchange generally varies depending on the existing (or anticipated future) traffic control devices at the intersection between the freeway ramp terminal and the crossroad. For example, where the freeway ramp terminal and crossroad are signalized, the relationship between safety and access spacing is based on the signal spacing. For other traffic control, the relationship between safety and access spacing may be a function of other parameters, such as unsignalized access spacing (intersections and driveways). However, where the ramp terminates as a free-flow merge or under yield control on a crossroad, the dynamics between the crash rate and spacing are more complex, owing to the various movements and operations involved. These include the merge where the ramp traffic enters the arterial and, for traffic turning left downstream, the weaving movement to enter the left lane and the transition into a left-turn lane.

Roadway Cross Section

Median Type (Raised, TWLTL, Undivided)

The research by Schultz, Braley, and Boschert in Correlating Access Management to Crash Rate, Severity, and Collision Type indicated that the presence of a raised median corresponded to a reduction of 1.23 crashes per MVMT.(11) In addition, raised medians were negatively correlated with right-angle collisions, while TWLTLs were positively correlated with opposite-direction collisions.

The research performed by Gluck, Levinson, and Stover for NCHRP Report 420, which was discussed in the Access Spacing section, also investigated the relationship between median type and crash rates.(3) Figure 7 (shown earlier in the Access Spacing section) illustrates the relationship developed between crash rate and roadway cross section.

The literature search in NCHRP Report 420 found that many studies had analyzed the safety benefits of installing TWLTLs or nontraversable medians on undivided highways and replacing TWLTLs with nontraversable medians. There were mainly two types of studies. Some studies (particularly those where TWLTLs or medians were installed on undivided highways) report results of before–after comparisons for a given facility. Other studies compared crash experience and rates on highways with different cross sections (i.e., medians versus TWLTLs).(3)

Both types of studies found that crash rates were reduced when TWLTLs or medians were introduced on undivided, multilane highways. Most studies and the models derived from them also suggest that safety is improved where nontraversable medians replace TWLTLs. NCHRP Report 420 found that, overall, TWLTLs had a 20-percent lower crash rate, and nontraversable medians had a 40-percent lower crash rate than undivided road sections. These patterns appeared to be consistent for all access density ranges.(3)

Bonneson and McCoy, in Capacity and Operational Effects of Midblock Left-Turn Lanes (NCHRP Report 395), presented procedures for estimating the safety (and operational) impacts of different midblock left-turn treatments.(6) They also included guidelines for selecting among nontraversable medians, TWLTLs, and undivided cross sections. They included a series of tables to estimate annual crash frequencies for ¼-mi road segments.

The research in NCHRP Report 395 compared the different outcomes from a number of crash prediction models developed by different researchers. A composite finding suggested that as traffic volumes exceed approximately 15,000 average daily traffic (ADT), a raised median is safer than a TWLTL. Both are safer than no median (i.e., an undivided roadway) for volumes at least as low as 10,000 ADT.(6)

Eisele and Frawley, in Estimating the Safety and Operational Impact of Raised Medians and Driveway Density: Experiences From Texas and Oklahoma Case Studies, investigated the relationship between access density and crash rate for raised median and nonraised median corridors separately.(10) The relationship was still positively correlated but was slightly steeper for the nonraised median corridors than for the raised median corridors. The researchers concluded that when the number of conflict points is reduced through introduction of a raised median, there are relatively lower crash rates (which result in a reduced slope of the regression line).

Hallmark and the other researchers indicated in the Toolbox to Assess Tradeoffs Between Safety, Operations, and Air Quality for Intersection and Access Management Strategies: Final Report that FHWA, in 2003, evaluated data from seven States and suggested that raised medians reduced crashes by more than 40 percent in urban areas and that a study of corridors in Iowa found that the use of TWLTLs reduced crashes by 70 percent.(13)

Median Opening Spacing

Kach, in The Comparative Accident Experience of Directional and Bi-Directional Signalized Intersections, analyzed the safety effects of replacing full-median openings with directional crossovers.(14) The mean intersection-related crash rates overall were about 15 percent lower for the directional crossovers. The corresponding rates for intersection-related injury crashes were about 30 percent lower for the directional crossovers.

Potts and the other researchers involved with the research for Safety of U-Turns at Unsignalized Median Openings (NCHRP Report 524) investigated the safety and operational effect of U-turns at unsignalized median openings.(7) The safety performance of typical median opening designs was documented, and guidelines for the use, location, and design of unsignalized median openings were developed. The research included unsignalized median openings on all types of divided highways, but the focus was urban/suburban arterials because these present the greatest current challenge to highway agencies with respect to AM. The following are among the research conclusions:

Property Access

Although property access strategies can reduce conflicts along the arterial, no literature was found that quantified the safety impacts of these strategies. Studies of the relationship between safety and property access strategies (i.e., frontage/backage roads and internal cross connectivity) could be complicated by an extensive roadway network that could be involved and require investigation. The analysis network would need to include the arterial from which the property has access, the larger network that would include the frontage/backage road (and intersections), as well as facilities that are used for cross connectivity. Relevant variables to consider include the configuration of the frontage/backage road, whether it is one-way or two-way, and the separation distances between it and the parallel arterial.

 

 

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