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    Designing for Safe MobilityA Holistic, User-Centric Approach to Roadway DesignKYTC’s Designing for Safe Mobility website houses critical knowledge roadway designers need to plan and design facilities that deliver on the promise of providing safe and efficient mobility for all users. Along with short primers on topics that inform contemporary design best practices, users can search […]

  • 6.2 Task Analysis of Curve Driving

    .2 Task Analysis of Curve Driving Designers must identify areas throughout curves where motorists have high workloads. This knowledge is useful for selecting design features that make driving tasks easer (e.g., clearer road delineation, wider lanes, longer radius) or where visual distractions should be eliminated. Some rules of thumb are listed below. Minimize visual workloads […]

  • 6.4 Influence of Perceptual Factors on Curve Driving

    .4 Influence of Perceptual Factors on Curve Driving Many motorists base curve speeds on their perception of road features and the apparent curve radius. The apparent curve radius is the radius as seen from the motorists’ perspective. Topography and road elements can distort the apparent radius, making it look sharper or flatter. This issue is […]

  • 6.6 Speed Selection on Horizontal Curves

    .6 Speed Selection on Horizontal Curves When establishing design speeds, account for functional/context classification, the legal speed limit, terrain type, anticipated vehicle volumes, level of surrounding development, expense, and design consistency. Procedure Determine the curve radius, superelevation, and offset distances from the lane’s center to visual obstructions. Calculate for different vehicle types the maximum possible […]

  • 6.8 Countermeasures for Improving Steering and Vehicle Control Through Curves

    .8 Countermeasures for Improving Steering and Vehicle Control Through Curves Motorists retain better control of vehicles when curves conform with their expectations and typical behaviors. Several design strategies promote safer alignments (Table 6.2).
    Table 6.2 Design Strategies to Improve Steering Control Through Curves
    Feature Guidance
    Curvature
    • Minimize the use of maximum allowable curvature for a given design speed.
    Spirals
    • Use spiral transition curves whenever possible, especially on roads with design speeds ≥ 60 mph.
    • The length of a spiral curve should equal the distance traveled during steering time (2 – 3 seconds, depending on the radius)
    • Recommended curve radius for two-lane roads with a speed limit of 50 mph is 393 – 755 ft. Clothoid parameters should be 0.33 b – .05 R.
    Reverse Curves
    • Do not use tangent sections in reverse curves when the distance between the first curve’s exit and the second curve’s entrance is short enough to establish a curved path through the tangent.
    • This is 262 ft or less for two-lane highways and 443 ft for freeways.
    Superelevation
    • Design the superelevation to result in zero lateral acceleration through a curve at design speed.
    Design Consistency
    • Avoid the use of sharp, isolated curves.
    • Maintain consistency in superelevation, road width, and other curve features.
    Do you have more questions? Get in touch! Chris VanDyke Research Scientist | Program Manager chrisvandyke@uky.edu

  • 6.10 Countermeasures to Improve Pavement Delineation

    .10 Countermeasures to Improve Pavement Delineation Good pavement surface markings allow drivers to (a) detect and recognize curves earlier, (b) reduce speeds appropriately, and (c) adjust their lateral position. Table 6.3 provides design guidance for markings. Table 6.4 lists the strengths of different markings.
    Table 6.3 Countermeasures to Improve Curve Detection and Motorist Performance
    Feature Guidance
    General
    • Pavement markings should have small gaps, long dashes, and short repetition cycles. Combining treatments increases their effectiveness.
    Edge Line and Centerline
    • Use edge lines on (a) sharp or frequent curves, (b) narrow roads, or (c) near crossing roads and major driveways.
    • Use the widest possible edge lines and centerlines to maximize their visibility.
    • If possible, use durable striping materials with high retroreflectivity.
    Raised Retroreflective Pavement Markers (RRPMs)
    • Combine RRPMs with edge lines and centerlines.
    • On sharp curves (≥ 12 degrees), use pairs of RRPMs on the centerline’s outside edges. On flatter curves, a single RRPM is sufficient.
    • Place RRPMs 787 ft upstream of a curve.
      • On sharp curves, space RRPMs at 131 ft intervals. On flat curves, space RRPMs at 262 ft intervals.
    Transverse Stripes
    • Implement transverse stripes as graduated rumble strips where possible.
    • Install stripes so that drivers experience them at 0.5-second intervals when they decelerate at the desired rate (e.g. 2.95 m/s2).
    SLOW Text with Arrow
    • In hazard-prone areas or locations with sharp curves, install a sign that reads SLOW and contains a directional arrow 230 ft upstream of the curve.
    Table 6.4 Effects of Different Marking Types
    Treatment Benefits
    General — Surface Markings
    • Strongest indication of downstream curvature
    • Provide short-range steering control cues (compensatory control)
    General — Post-Mounted Chevrons
    • Strongest guidance cues
    • Provide long-range guidance (anticipatory control)
    Combined Treatments
    • More effective than single treatments
    Edge Line / Centerline
    • Best for curve recognition, curve perception, and reduction of lateral variability
    • Discontinuities in edge lines help motorists detect downstream features such as driveways and intersections
    RRPMs
    • Improved visibility of edge lines and centerlines
    • Reduce lane encroachments
    • Visual and rumble effects let motorists know they are encroaching on the shoulder or adjacent lane
    Transverse Stripes
    • Reduced speeds
    • May be most effective when combined with rumble strips
    SLOW Text with Arrow
    • Reduced speeds
    • Warns motorists of downstream curves
    Do you have more questions? Get in touch! […]

  • 6.12 Signs on Horizontal Curves

    .12 Signs on Horizontal Curves Use curve warning signs to communicate downstream changes in horizontal alignment to motorists. Signs need to include a directional arrow that indicates if a curve bends to the left or right. Advisory speed signs can also be used. Table 6.5 contains placement guidelines for curve warning signs based on the […]

  • 5.2 Key Components of Sight Distance

    .2 Key Components of Sight Distance Formally, sight distance is the length of road a vehicle travels before the driver completes a maneuver in response to an element, hazard, or condition that requires a change in the vehicles speed and/or path. Sight distance consists of two components — perception-reaction time (PRT) and the amount of […]

  • 5.4 Determining Stopping Sight Distance

    .4 Determining Stopping Sight Distance Stopping sight distance is the distance a motorist traveling at a particular speed needs to stop their vehicle once they detect an object on the road. To determine stopping sight distance, use actual operating speeds if they are available — not the design speed. The equation below is used to […]

  • 5.6 Determining Intersection Sight Distance

    .6 Determining Intersection Sight Distance Drivers must have a clear view of an entire intersection so they can see traffic control devices and see far enough upstream or downstream along an intersecting road to identify acceptable gaps in cross traffic before pulling onto the intersecting road. All quadrants of an intersection must have a triangular […]