Application of Crash Modification Factors for Access Management, Volume 1: Practitioner's Guide (2021) / Chapter Skim
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Pages 10-49
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10 Access management 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.
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Safety Effects of Access Management 11 Table 4. Overview of property access strategies.
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12 Application of Crash Modification Factors for Access Management quantifying segment- and intersection-level safety performance. Chapter 5 presents methods for quantifying corridor-level safety performance.
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Safety Effects of Access Management 13 The Highway Safety Manual (1st Edition) provides separate models for multivehicle-driveway and non-driveway crashes per mile, considering the traffic volume, number and type of driveways, and median type (divided or undivided)
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14 Application of Crash Modification Factors for Access Management Strategy Crash Severity CMF CMF ID Applicability Change driveway density from X to Y driveways per mile All exp(0.0096*
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Safety Effects of Access Management 15 "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.
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16 Application of Crash Modification Factors for Access Management Source: Adapted from Gluck et al. 1999, Figure 5, p.
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Safety Effects of Access Management 17 Functional Area and 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 provide corner clearance by locating driveways outside the functional area of an intersection.
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18 Application of Crash Modification Factors for Access Management Based on a review of the CMF Clearinghouse in December 2019, there were no CMFs rated three stars or higher that apply to the functional area of an intersection. There were 28 CMFs rated three stars or higher that apply to corner clearance at signalized intersections on urban and suburban arterials.
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Safety Effects of Access Management 19 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.
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20 Application of Crash Modi cation Factors for Access Management results are not statistically signi cant, even at the 90-percent con dence level (i.e., CMF = 2.12 with standard error = 0.91)
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Safety Effects of Access Management 21 vehicles have been shown to account for approximately 72 percent of crashes at a driveway (FHWA 2010)
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22 Application of Crash Modification Factors for Access Management Strategy Crash Severity CMF CMF ID Applicability Install non-traversable median on undivided road Fatal and injury 0.61 21 2-lane roads Injury 0.78 22 Multilane roads PDO 1.09 23 Multilane roads All 0.29 2219 1,390 to 51,200 vehicles/day All 0.86 3935 2-lane roads < 45 mph posted speed Note: The base condition is an undivided road. Source: FHWA n.d.
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Safety Effects of Access Management 23 Median Opening Spacing and Design The Florida Department of Transportation's Median Handbook indicates that "a restrictive median with well-designed median openings is one of the most important tools" to improve the safety and efficiency of the highway system (Florida DOT 2014)
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24 Application of Crash Modification Factors for Access Management most driveways along the corridor become right-in/right-out driveways; however, the left-turn movements that would have been allowed with a full median opening are now shifted to a downstream U-turn location. As such, it is important to consider the net safety performance when managing turning movements at a specific location.
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Safety Effects of Access Management 25 Median Treatment -- Two-Way Left-Turn Lane Like installations of non-traversable medians on formerly undivided highways, installations of TWLTLs offer safety and operational benefits. Compared to undivided highways, TWLTLs allow the deceleration and storage of left-turning vehicles outside of the through-traffic lanes.
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26 Application of Crash Modification Factors for Access Management of arterials with and without TWLTLs, as discussed in Chapter 4 of this guide; however, the results should not be used to infer CMFs. In some cases, it may be reasonable to use the predictions from two different models to estimate the change in safety (i.e., infer CMFs)
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Safety Effects of Access Management 27 Strategy Crash Severity CMF CMF ID Applicability Install TWLTL on 4-lane undivided road All 0.48 9416 • 6,800 to 27,304 vehicles/day• Non-intersection crashes All 0.36 9417 • 18,748 to 20,098 vehicles/day • Population of < 2,500 people • Non-intersection crashes All 0.56 9418 • 6,800 to 24,880 vehicles/day • Population of 5,000 to 25,000 people • Non-intersection crashes All 0.66 9419 • 22,262 to 24,880 vehicles/day • Population of 50,000 to 100,000 people • Non-intersection crashes All 0.21 9420 • 24,880 to 27,304 vehicles/day • Population of 100,000 to 200,000 people • Non-intersection crashes All 0.43 9421 • 24,880 to 27,304 vehicles/day • Population of 200,000 to 500,000 people • Non-intersection crashes All -0.0006*
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28 Application of Crash Modification Factors for Access Management FHWA's Pedestrian Safety Guide and Countermeasure Selection System (Zegeer et al.
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Safety Effects of Access Management 29 A review of the CMF Clearinghouse, in December 2019, indicated that there were eight CMFs rated three stars or above that apply to the conversion of urban and suburban arterials from two-way to one-way operation. Table 28 presents three related CMFs along with the general applicability, CMF ID (linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on "NCHRP Research Report 974")
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30 Application of Crash Modification Factors for Access Management treatment is shown for comparison purposes. NCHRP Report 745: Left-Turn Accommodations at Unsignalized Intersections (Fitzpatrick et al.
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Safety Effects of Access Management 31 Beyond the Highway Safety Manual (1st Edition) , the CMF Clearinghouse presents several other CMFs rated three stars or higher that apply to left-turn lanes at intersections on urban and suburban arterials.
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32 Application of Crash Modification Factors for Access Management Strategy Crash Severity CMF CMF ID Applicability Install left-turn lane All 0.75 7996 • 3-legged signalized intersections • 2-lane suburban roads • 2,981 to 18,248 vehicles/day on major road • 972 to 13,880 vehicles/day on minor road Fatal and injury 0.57 7999 All 0.92 7997 • 4-legged signalized intersections • 2-lane suburban roads • 1,360 to 17,566 vehicles/day on major road • 746 to 8,884 vehicles/day on minor road Fatal and injury 0.80 8000 All 0.88 7998 • 3- and 4-legged signalized intersections • 2-lane suburban roads • 1,360 to 18,248 vehicles/day on major road • 746 to 13,880 vehicles/day on minor road Fatal and injury 0.74 8001 Note: The base condition is no left-turn lanes present. Source: FHWA n.d.
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Safety Effects of Access Management 33 Right-Turn Lanes Right-turn movements, especially those that are made from shared lanes, create conflict points and delays. Right-turn lanes provide a refuge for right-turning vehicles by removing those vehicles from the through-traffic lane(s)
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34 Application of Crash Modification Factors for Access Management suburban arterials. Based on a review of the CMF Clearinghouse in December 2019, Table 34 presents two related CMFs along with the general applicability, CMF ID (linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on "NCHRP Research Report 974")
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Safety Effects of Access Management 35 • Double crossover diamond interchange (diverging diamond) , • Displaced left-turn interchange (continuous flow intersection)
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36 Application of Crash Modification Factors for Access Management Based on a review of the CMF Clearinghouse in December 2019, there were several CMFs rated three stars or higher, including those in Table 36, that apply to alternative intersection designs on urban and suburban arterials. Refer to the previous section, Median Treatment -- Non-Traversable Median, for CMFs related to a variation of the median U-turn intersection (i.e., conversion of direct left turns to right turns followed by U-turns)
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Safety Effects of Access Management 37 Strategy Crash Severity CMF CMF ID Applicability Convert signalized intersection to single-lane or multilane roundabout All 0.79 4184 • Urban and suburban • 1 or 2 circulating lanes • 3- and 4-legged intersections • 15–35 mph posted speed • 5,300 to 52,500 vehicles/day on major road Fatal and injury 0.34 4185 All 0.96 4934 • 4-lane roads • 3- and 4-legged intersections • Signalized intersections • 4,100 to 48,100 entering vehicles/day Fatal and injury 0.35 4935 All 1.92 5523 • 1 to 3 circulating lanes • 3- and 4-legged intersections • Signalized intersections • 5,700 to 32,900 vehicles/day on major road • 1,900 to 24,200 vehicles/day on minor road All 0.58 4186 • Suburban • 1 or 2 circulating lanes • 3- and 4-legged intersections • 15–35 mph posted speed • 5,300 to 52,500 vehicles/day on major road Fatal and injury 0.26 4187 All 1.15 4188 • Urban • 1 or 2 circulating lanes • 3- and 4-legged intersections • 15–35 mph posted speed • 5,300 to 52,500 vehicles/day on major road Fatal and injury 0.45 4189 All 1.07 4190 • Urban and suburban • 1 or 2 circulating lanes • 3-legged intersections • 15–35 mph posted speed • 5,300 to 52,500 vehicles/day on major road Fatal and injury 0.37 4191 All 0.76 4192 • Urban and suburban • 1 or 2 circulating lanes • 4-legged intersections • 15–35 mph posted speed • 5,300 to 52,500 vehicles/day on major road Fatal and injury 0.34 4193 Convert signalized intersection to single-lane roundabout All 0.74 4196 • 3- and 4-legged intersections • Signalized intersections • 15–35 mph • 5,300 to 52,500 vehicles/day on major road Fatal and injury 0.45 4197 Convert signalized intersection to multilane roundabout All 0.81 4194 • 3- and 4-legged intersections • Signalized intersections • 15–35 mph • 5,300 to 52,500 vehicles/day on major road Fatal and injury 0.29 4195 Note: The base condition is a conventional signalized intersection. Source: FHWA n.d.
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38 Application of Crash Modification Factors for Access Management 47 CMFs from the CMF Clearinghouse along with the general applicability, CMF ID (linked to the CMF Clearinghouse in the PDF of this publication, which can be found on www.trb.org by searching on "NCHRP Research Report 974") , and base condition.
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Safety Effects of Access Management 39 the intersecting primary roadway) , is not included because of the implications it has for traffic circulation patterns on the surrounding roadway network.
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40 Application of Crash Modification Factors for Access Management As indicated in FHWA's Access Management in the Vicinity of Intersections: Technical Summary (FHWA 2010) , driveway connections to public roads should be adequately designed to improve the safety and efficiency of vehicle movements to and from the roadway, while balancing safety with mobility interests.
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Safety Effects of Access Management 41 in consideration. Based on the application of information in AASHTO's A Policy on Geometric Design of Highways and Streets, the sight distance needed under various assumptions of physical conditions and driver behavior is directly related to vehicle speeds and to the resultant distances traversed during perception-reaction time and braking (AASHTO 2018)
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42 Application of Crash Modification Factors for Access Management Source: Eccles et al.
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Safety Effects of Access Management 43 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 access management strategy to provide direct access to properties and separate through traffic from local access-related traffic.
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44 Application of Crash Modification Factors for Access Management Shared Driveways and Internal Cross Connectivity Access management promotes the implementation of shared-access driveways and cross-access easements between adjacent properties, where possible; these allow pedestrians and vehicles to circulate between properties without reentering the abutting roadway (see Figure 15)
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Safety Effects of Access Management 45 Counterintuitive Results (Do Not Use) While the prior sections present numerous CMFs for estimating the safety effects of access management strategies, there are some results in the literature that are counterintuitive and require additional research before the results could be used with confidence.
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46 Application of Crash Modi cation Factors for Access Management Median Treatment -- Non-Traversable Median e Highway Safety Manual (1st Edition) provides models to estimate the safety performance of arterials with and without medians as discussed in Chapter 4 of this guide; however, the results should not be used to infer CMFs.
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Safety Effects of Access Management 47 Strategy Crash Severity CMF Applicability Install nontraversable median on 4lane undivided road All 1.18 • Traffic volume of 5,000 vehicles per day • Driveway density of 20 residential driveways per mile 0.81 • Traffic volume of 15,000 vehicles per day • Driveway density of 20 residential driveways per mile 0.68 • Traffic volume of 25,000 vehicles per day • Driveway density of 20 residential driveways per mile 1.38 • Traffic volume of 5,000 vehicles per day • Driveway density of 40 residential driveways per mile 0.95 • Traffic volume of 15,000 vehicles per day • Driveway density of 40 residential driveways per mile 0.80 • Traffic volume of 25,000 vehicles per day • Driveway density of 40 residential driveways per mile Note: These inferred CMFs SHOULD NOT BE USED to quantify the safety effect of installing non-traversable medians on undivided roads. Source: NCHRP Project 17-62.
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48 Application of Crash Modi cation Factors for Access Management densities; this also contradicts conventional wisdom. is appears to be another case where cross-sectional comparison will not give reasonable and reliable CMFs because the SPF comparisons do not account for all differences in safety between the two site types even a er the SPFs are calibrated to the same spatial and temporal conditions.
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Safety Effects of Access Management 49 e CMFs from the CMF Clearinghouse are more reliable for estimating the safety impacts of this strategy. Intersection Treatments ere were no counterintuitive ndings associated with intersection treatments, including le -turn lanes, right-turn lanes, and alternative intersection designs.
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