THE FUTURE OF THE TXDOT ROADWAY DESIGN MANUAL Kenneth Mora, P.E. (Design Division) 10/10/2017
Table of contents 1 2 Reduction in FHWA design controlling criteria Innovative Intersection Guidance 3-7 8-42 3 Right-turn Slip Lane Guidance 43-45 4 5 6 HOV Guidance DHV Calculation Conclusion 46-47 48-49 50 2
FHWA DESIGN CONTROLLING CRITERIA (HIGH SPEED) REDUCTION IN FHWA DESIGN CONTROLLING CRITERIA, RESULTING FROM RESEARCH REPORT NCHRP 783, WHICH USED THE HCM AND HSM TO EXAMINE EXISTING CONTROLLING CRITERIA NHS (HIGH SPEED): Reduced from 13 to 10 FHWA controlling criteria. 3 REMOVED FHWA CONTROLLING CRITERIA: Lateral Offset to Obstruction: Already covered under shoulder width in rural areas. Most relevant to urban areas (mirrors or other appurtenances of heavy vehicle striking roadway objects & passengers in parked cars able to open doors), but did not rise to same level of effect as other controlling criteria. 3
FHWA DESIGN CONTROLLING CRITERIA (HIGH SPEED) Vertical Alignment: - Crest Vertical Curve design is already covered under SSD. - Grade is already explicitly covered as a separate criteria. - Sag vertical curves, research determined no relationship to the effects on crashes. Generally, sag vertical curve length is not critical in day time when drivers can see beyond the sag vertical curve; nor at night when vehicle taillights and headlights make another vehicle visible in or beyond a sag vertical curve. Bridge Width: Research determined/surmised little relationship in crash frequency. Lane and shoulder width apply to bridges, so this is already covered by other controlling criteria. 4
FHWA DESIGN CONTROLLING CRITERIA (HIGH SPEED) EXISTING DESIGN CONTROLLING CRITERIA Design Speed Lane Width Shoulder Width Structural Capacity Horizontal Alignment Grades Stopping Sight Distance Cross Slope Superelevation Vertical Clearance Bridge Width Vertical Alignment Lateral Offset to obstruction A. UPDATED FHWA DESIGN CONTROLLING CRITERIA (NHS) Design Speed Lane Width Shoulder Width Design Loading Structural Capacity Horizontal Curve Radius Maximum Grade Stopping Sight Distance (excluding sag curves) Cross Slope Superelevation (rate only) Vertical Clearance 5
FHWA DESIGN CONTROLLING CRITERIA (LOW SPEED) NHS (LOW SPEED): Only Design Speed and Design Loading Structural Capacity would remain as FHWA controlling criteria. Research indicates that the current controlling criteria are less influential on the traffic operational and safety performance of low-speed urban and suburban arterials than other features such as intersection design and access management strategies. 6
FHWA DESIGN CONTROLLING CRITERIA (LOW SPEED) EXISTING DESIGN CONTROLLING CRITERIA Design Speed Lane Width Shoulder Width Structural Capacity A. UPDATED FHWA DESIGN CONTROLLING CRITERIA (NHS) Design Speed Design Loading Structural Capacity Horizontal Alignment Grades Stopping Sight Distance Cross Slope Superelevation Vertical Clearance Bridge Width Vertical Alignment Lateral Offset to obstruction 7
INNOVATIVE INTERSECTION GUIDANCE Proposed Updates to TxDOT Roadway Design Manual TxDOT is currently updating the RDM to incorporate guidance for innovative intersections: New Appendix E. RDM Updates will include general guidance for: Roundabouts, DDI s, MUT s, RCUT s, and DLT s. 8
INNOVATIVE INTERSECTION GUIDANCE (GENERAL) What Are Innovative Intersection Designs? Designs that eliminate, relocate, or modify conflict points by improving the way traffic makes certain movements. Safety Mobility Value Fewer Conflict Points Significant Before and After Crash Reductions and Severity (multimodal) Reduced Delay Reduced Congestion Typically Less ROW needed Decreased Construction Costs Quicker Implementation 9
INNOVATIVE INTERSECTION GUIDANCE (ROUNDABOUT) What is a Roundabout? A Roundabout is a type of circular intersection with yield control of entering traffic, islands on the approaches and appropriate roadway curvature to reduce vehicle speeds. Why consider a Roundabout? Improves Safety Reduces Congestion Reduces pollution and fuel use Saves money 10
INNOVATIVE INTERSECTION GUIDANCE (ROUNDABOUT) Improve safety Fewer Conflict Points More than 90% reduction in fatalities 76% reduction in injuries 35% reduction in all crashes Slower speeds are generally safer for pedestrians, 30-40 percent reduction in pedestrian incidents 10 percent reduction in bicycle crashes 30-50 percent increase in traffic capacity With roundabouts, head-on and high-speed right angle collisions are virtually eliminated. [Traditional Intersection] Red dots indicate 32 vehicle-tovehicle conflict points in a standard four-way intersection. Potential Vehicle Conflict Points [Roundabout] Red dots indicate eight vehicleto-vehicle conflict points in a modern roundabout. 11
INNOVATIVE INTERSECTION GUIDANCE (ROUNDABOUT) TxDOT updates to Roadway Design Manual Incorporating Roundabout Guidance Primary Sources: NCHRP 672 (Roundabouts an Informational Guide, 2 nd Edition). NCHRP Synthesis 488 Roundabout Practices. Consideration for Roundabouts for locations that meet or nearly meet signal warrants; and proposed all-way stop control intersections. Urban, Suburban, and Rural Applications Typical Design Vehicle WB-67 12
INNOVATIVE INTERSECTION GUIDANCE (ROUNDABOUT) TxDOT updates to Roadway Design Manual Incorporating Roundabout Guidance MOUNTABLE CENTRAL ISLAND ICD TRUCK APRON SPLITTER ISLAND SPLITTER ISLAND MINI-ROUNDABOUT SINGLE-LANE ROUNDABOUT TWO-LANE ROUNDABOUT 13
INNOVATIVE INTERSECTION GUIDANCE (ROUNDABOUT) TYPICAL ROUNDABOUT DESIGN SPEED AND DIAMETERS 14
INNOVATIVE INTERSECTION GUIDANCE (ROUNDABOUT) Single-lane roundabouts designed for low speed operation are one of the safest treatments available for at-grade intersections. Drivers have no lane use decisions to make. Pedestrians cross one lane of traffic at a time. Roadway speeds and widths are low enough to allow comfortable mixed bicycle and motor vehicle flow. (NCHRP Report 672) 15
INNOVATIVE INTERSECTION GUIDANCE (ROUNDABOUT) TxDOT One-lane vs. Two-lane Roundabouts The preference in Texas is to maintain a single-lane roundabout as long as possible. If a single-lane roundabout is adequate for up to 10 years after the opening year, a single-lane roundabout should be constructed. If a multi-lane roundabout is required after 10 years, the single-lane roundabout should be constructed having the footprint of a multi-lane roundabout, and be designed to be easily retrofitted to a multi-lane roundabout when needed. 16
INNOVATIVE INTERSECTION GUIDANCE (ROUNDABOUT) Example of accelerated construction, low-cost roundabout 17
INNOVATIVE INTERSECTION GUIDANCE (ROUNDABOUT) SR-11/SR-124/Galilee Church Road (Jackson Co., Georgia) Mini-roundabout (90 ICD) Quick construction $63,000 construction costs Reduced crashes Reduced ¼ mi. stopped queue to 8 10 car rolling queue 18
INNOVATIVE INTERSECTION GUIDANCE (DDI) What is a Diverging Diamond Interchange (DDI)? A DDI is a type of intersection that connects a freeway with a major highway. It is based on a modified diamond intersection with a shift in traffic within the intersection to safely and efficiently accommodate high volume left turn movements. Traffic within the intersection briefly drives on the left side of the road to allow left turn movements to occur without crossing oncoming traffic or stopping. How do pedestrians use a DDI? Pedestrians use signalized pedestrian crossings and then may be directed to a center pedestrian island in the middle of the road. Why consider a DDI? Improves Safety; Improves capacity with two phase signals; Reduces pollution and fuel use; saves money; efficient during both peak hours and other times. 19
INNOVATIVE INTERSECTION GUIDANCE (DDI) DDI in Idaho (IH-86 & US 91) 20
INNOVATIVE INTERSECTION GUIDANCE (DDI) Improves safety Fewer conflict points 60% reduction in all crashes and injuries Slower speeds are generally safer for pedestrians, 30-40 percent reduction in pedestrian incidents This is a relatively new interchange concept; therefore the statistics on the reduction in crashes and the increase in traffic capacity varies by location. Pink dots indicate 10 crossing conflict points, 8 merging conflict points and 8 diverging Pink dots indicate two crossing conflict points, 6 merging conflict points and 6 diverging 21
INNOVATIVE INTERSECTION GUIDANCE (DDI) KEY CHARACTERISTICS OF A DDI 22
INNOVATIVE INTERSECTION GUIDANCE (DDI) TxDOT updates to Roadway Design Manual Incorporating DDI Guidance Primary Sources: FHWA DDI Informational Guide Typical Design Speed Range from 25-35 mph through two intersection crossovers Typical Design Vehicle WB-67 Relatively flat in order to increase driver sight distance 23
INNOVATIVE INTERSECTION GUIDANCE (DDI) CROSSOVER GEOMETRY (CURVE ANGLE) CROSSOVER GEOMETRY (CURVE RADII) 24
INNOVATIVE INTERSECTION GUIDANCE (DDI) PED. Ped crossing of only one direction of traffic allows reduction in ped conflict points Ped crossing down center allows wide walkway Two-phase signalization benefits ped crossing time 25
INNOVATIVE INTERSECTION GUIDANCE (MUT) What is a Median U-Turn (MUT)? A MUT is an intersection form that replaces direct left turns at an intersection with indirect left turns using a U-turn movement in a wide median. The MUT eliminates left turns on both intersecting streets. Why consider a MUT? Reduces conflict points for both vehicles and pedestrians Reduces traffic signal phases, thus improves intersection operations General increase in safety performance 26
INNOVATIVE INTERSECTION GUIDANCE (MUT) EXAMPLE OF A MUT INTERSECTION 27
INNOVATIVE INTERSECTION GUIDANCE (MUT) MAJOR STREET LEFT-TURN MOVEMENT MINOR STREET LEFT-TURN MOVEMENT RIGHT-TURN MOVEMENT 28
INNOVATIVE INTERSECTION GUIDANCE (MUT) TxDOT updates to Roadway Design Manual Incorporating MUT Guidance Primary Sources: FHWA MUT Informational Guide Typical spacing for Median Opening to main intersection is 400 to 600 ft. based on signal timing Typical Design Vehicle WB-67 29
INNOVATIVE INTERSECTION GUIDANCE (MUT) MINIMUM MEDIAN WIDTH MUT WITH A LOON TO ACCOMMODATE DESIGN VEHICLE LOON 30
INNOVATIVE INTERSECTION GUIDANCE (MUT) PED. Reduction in ped conflict points Peds don t have to contend with left turn vehicle movements Two-phase signalization benefits 31
INNOVATIVE INTERSECTION GUIDANCE (RCUT) What is a Restricted Crossing U-Turn (RCUT)? An RCUT is an at-grade intersection with directional medians such that the minor road traffic must turn right and make a U-turn back to cross or make a left-turn maneuver. Why consider an RCUT? Significant reduction in conflict points Reduction in crash rates and crash severity Can improve traffic flow At signalized; or unsignalized intersections for rural locations with low ped activity. 32
INNOVATIVE INTERSECTION GUIDANCE (RCUT) RCUT WITH SIGNAL CONTROL RCUT WITH STOP CONTROL 33
INNOVATIVE INTERSECTION GUIDANCE (RCUT) TxDOT updates to Roadway Design Manual Incorporating RCUT Guidance Primary Sources: FHWA RCUT Informational Guide Typical spacing for Median turnaround to main intersection is 400 to 800 ft. Typical Design Vehicle WB-67 34
INNOVATIVE INTERSECTION GUIDANCE (RCUT) RCUT SPACING CONSIDERATION FOR A MINOR STREET THROUGH OR LEFT MOVEMENT. 35
INNOVATIVE INTERSECTION GUIDANCE (RCUT) MINIMUM MEDIAN WIDTH RCUT WITH A LOON TO ACCOMMODATE DESIGN VEHICLE LOON 36
INNOVATIVE INTERSECTION GUIDANCE (RCUT) PED. Reduction in ped conflict points Reduction in cycle lengths (when signalized) RCUT is better suited for relatively low ped activity 37
INNOVATIVE INTERSECTION GUIDANCE (DLT) What is a Displaced Left Turn Intersection (DLT)? A DLT is an intersection form that relocates one or more left turn movements on an approach to the other side of the opposing traffic. This allows left-turn movements to proceed simultaneously with the through movements and eliminates the left-turn phase for this approach. Why consider a DLT? Reduces conflict points More green time for major movements offering better progression Compatible with high-volume turning movements 38
INNOVATIVE INTERSECTION GUIDANCE (DLT) EXAMPLE OF A DLT INTERSECTION 39
INNOVATIVE INTERSECTION GUIDANCE (DLT) Primary Sources: FHWA DLT Informational Guide Typical distance between main intersection and crossovers ranges from 300 500 ft. Typical Design Vehicle WB-67 40
INNOVATIVE INTERSECTION GUIDANCE (DLT) Vehicle path through main intersection will determine curb line, stop bar line, and width. Goal is to provide smooth alignment for traffic and avoid back to back reverse curves. 41
INNOVATIVE INTERSECTION GUIDANCE (DLT) PED. Crosswalks at same locations as a conventional intersection Median used to provide a refuge for 2-stage ped crossing 42
RIGHT-TURN SLIP LANE GUIDANCE (APP. D) What is a Right-Turn Slip Lane? A right-turn slip lane is a form that separates right-turning traffic from adjacent lanes and allows higher speed right turns. The right-turn slip lane also reduces pedestrian exposure by allowing them to cross the roadway in two stages. 43
RIGHT-TURN SLIP LANE GUIDANCE TxDOT updates to Roadway Design Manual Incorporating Right-turn Slip Lane Guidance Applications for Urban, Suburban, or Rural roadways Recommended angle of entry 70 degrees Curb radius and curb-to-curb width designed to accommodate larger design vehicles; striping used to delineate path for smaller vehicles Crosswalk located near middle of channelized island 44
RIGHT-TURN SLIP LANE GUIDANCE RIGHT-TURN SLIP LANE W/ DECEL LANE RIGHT-TURN SLIP LANE W/OUT DECEL LANE 45
HOV LANE GUIDANCE (APP. F) AASHTO Guide for High-Occupancy Vehicle Facilities, 3rd Edition (2004) TTI study (2005) shows use of positive barriers significantly decreased crash rates NCHRP Report 835 (2016) Guidelines for Implementing Managed Lanes HOV/HOT lanes HOT drivers pay toll to use HOV 85 th Legislative Session SB 312 limits HOT implementation on future projects Texas Transportation Code Chapter 228 85 th Legislative Session Pave it Forward summary of changes impacting TxDOT 46
HOV LANE GUIDANCE (APP. F) SB 312 prevents TxDOT from adding a toll component after Sept. 1, 2017, to any currently non-tolled HOV lanes, unless the project meets at least one of the remaining four exceptions found in Section 228.201 (1-4), Texas Transportation Code. The new limitations imposed by SB 312 do not apply to a highway or segment of highway being operated as a toll project by TxDOT or an entity under contract with TxDOT before Sept. 1, 2017, or to a project included in the State s air quality state implementation plan before Sept. 1, 2017. 47
CALCULATION OF DESIGN HOUR VOLUME (DHV) TxDOT updates to Roadway Design Manual clarifying DHV computation Average Daily Traffic (ADT): Represents the total traffic for a year divided by 365. Design Hourly Volume (DHV): typically the 30 th highest hourly volume for the design year. K factor: Percent of ADT occurring in the design hour. Directional Distribution (D): Percentage of design hourly volume that is in the predominant direction of travel. T : Percentage of trucks during the design hour Truck Equivalency Factor (Et): Adjusts for additional impacts of trucks on traffic operations. 48
CALCULATION OF DESIGN HOUR VOLUME (DHV) Computation of DHV with non-directional ADT provided ADT=6030 vpd; K = 14%; D = 60%; T = 10%; Et =2 DHV = (ADT)(K)(D){1 + T(Et 1)} DHV = 6030*.14*.60*{1+.10(2-1)} = 557 passenger cars per hour in predominant direction. Computation of DHV with directional ADT provided ADT=3015 vpd; K = 14%; D = 60%; T = 10%; Et =2 DHV = 2(ADT)(K)(D){1 + T(Et 1)} DHV = 2*3015*.14*.60*{1+.10(2-1)} = 557 passenger cars per hour in predominant direction. 49
THANK YOU! Kenneth Mora, P.E. (Design Division) 10/10/2017