What’s in the roadway design manual - Texas A&M University · THE FUTURE OF THE TxDOT ROADWAY DESIGN MANUAL 2017 Short Course ... TxDOT updates to Roadway Design Manual Incorporating

THE FUTURE OF THE TxDOT ROADWAY DESIGN MANUAL 2017 Short Course

THE FUTURE OF THE TXDOT ROADWAY DESIGN MANUAL

Kenneth Mora, P.E. (Design Division)

10/10/2017


Table of contents

2

8-42

43-45

46-47

48-49

50

3-7Reduction in FHWA design controlling criteria

Innovative Intersection Guidance

Right-turn Slip Lane Guidance

HOV Guidance

DHV Calculation

Conclusion

1

2

3

4

5

6


FHWA DESIGN CONTROLLING CRITERIA (HIGH SPEED)

3

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.



4

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.



5

EXISTING DESIGN

CONTROLLING

CRITERIA

A. UPDATED FHWA DESIGN

CONTROLLING CRITERIA

(NHS)

Design Speed Design Speed

Lane Width Lane Width

Shoulder Width Shoulder Width

Structural Capacity Design Loading Structural

Capacity

Horizontal Alignment Horizontal Curve Radius

Grades Maximum Grade

Stopping Sight

Distance

Stopping Sight Distance

(excluding sag curves)

Cross Slope Cross Slope

Superelevation Superelevation (rate only)

Vertical Clearance Vertical Clearance

Bridge Width

Vertical Alignment

Lateral Offset to

obstruction


FHWA DESIGN CONTROLLING CRITERIA (LOW SPEED)

6

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.


FHWA DESIGN CONTROLLING CRITERIA (LOW SPEED)

7

EXISTING DESIGN

CONTROLLING

CRITERIA

A. UPDATED FHWA DESIGN

CONTROLLING CRITERIA

(NHS)

Design Speed Design Speed

Lane Width

Shoulder Width

Structural Capacity Design Loading Structural Capacity

Horizontal Alignment

Grades

Stopping Sight Distance

Cross Slope

Superelevation

Vertical Clearance

Bridge Width

Vertical Alignment

Lateral Offset to

obstruction


INNOVATIVE INTERSECTION GUIDANCE

8

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.


INNOVATIVE INTERSECTION GUIDANCE (GENERAL)

9

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 (multi-

modal)

• Reduced Delay

• Reduced Congestion

• Typically Less ROW

needed

• Decreased

Construction Costs

• Quicker

Implementation


INNOVATIVE INTERSECTION GUIDANCE (ROUNDABOUT)

10

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



11

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] [Roundabout]

Potential Vehicle Conflict Points

Red dots indicate eight vehicle-

to-vehicle conflict points in a

modern roundabout.

Red dots indicate 32 vehicle-to-

vehicle conflict points in a

standard four-way intersection.



12

TxDOT updates to Roadway Design Manual Incorporating

Roundabout Guidance

Primary Sources: NCHRP 672 (Roundabouts an Informational

Guide, 2nd 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



13


Roundabout Guidance

SINGLE-LANE ROUNDABOUT TWO-LANE ROUNDABOUTMINI-ROUNDABOUT

SPLITTER

ISLAND

TRUCK

APRON

MOUNTABLE

CENTRAL

ISLAND

SPLITTER

ISLAND

ICD



14

TYPICAL ROUNDABOUT DESIGN SPEED AND DIAMETERS



15

“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)



16

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.



17

Example of accelerated construction, low-cost roundabout



18

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


INNOVATIVE INTERSECTION GUIDANCE (DDI)

19

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.



20

DDI in Idaho (IH-86 & US 91)



21

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



22

KEY CHARACTERISTICS OF A DDI



23

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


Relatively flat in order to increase driver sight distance



24

CROSSOVER GEOMETRY (CURVE ANGLE) CROSSOVER GEOMETRY (CURVE RADII)


INNOVATIVE INTERSECTION GUIDANCE (DDI) – PED.

25

• 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


INNOVATIVE INTERSECTION GUIDANCE (MUT)

26

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



27

EXAMPLE OF A MUT INTERSECTION



28

MAJOR STREET LEFT-TURN MOVEMENT MINOR STREET LEFT-TURN MOVEMENT RIGHT-TURN MOVEMENT



29

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




30

MINIMUM MEDIAN WIDTH MUT WITH A LOON TO ACCOMMODATE DESIGN VEHICLE

LOON


INNOVATIVE INTERSECTION GUIDANCE (MUT) –PED.

31

• Reduction in ped conflict points

• Peds don’t have to contend with

left turn vehicle movements

• Two-phase signalization benefits


INNOVATIVE INTERSECTION GUIDANCE (RCUT)

32

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.



33

RCUT WITH SIGNAL CONTROL RCUT WITH STOP CONTROL



34

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.




35

RCUT SPACING CONSIDERATION FOR A MINOR STREET THROUGH OR

LEFT MOVEMENT.



36

MINIMUM MEDIAN WIDTH RCUT WITH A LOON TO ACCOMMODATE DESIGN

VEHICLE

LOON


INNOVATIVE INTERSECTION GUIDANCE (RCUT) –PED.

37

• Reduction in ped conflict points

• Reduction in cycle lengths (when

signalized)

• RCUT is better suited for relatively

low ped activity


INNOVATIVE INTERSECTION GUIDANCE (DLT)

38

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



39

EXAMPLE OF A DLT INTERSECTION



40

• Primary Sources: FHWA DLT

Informational Guide

• Typical distance between main

intersection and crossovers

ranges from 300 – 500 ft.

• Typical Design Vehicle WB-67



41

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.


INNOVATIVE INTERSECTION GUIDANCE (DLT) –PED.

42

• Crosswalks at same locations as

a conventional intersection

• Median used to provide a refuge

for 2-stage ped crossing


RIGHT-TURN SLIP LANE GUIDANCE (APP. D)

43

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.


RIGHT-TURN SLIP LANE GUIDANCE

44


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


RIGHT-TURN SLIP LANE GUIDANCE

45

RIGHT-TURN SLIP LANE W/ DECEL LANE RIGHT-TURN SLIP LANE W/OUT DECEL LANE


HOV LANE GUIDANCE (APP. F)

46

• 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

• 85th Legislative Session SB 312 limits “HOT” implementation on

future projects

• Texas Transportation Code – Chapter 228

• 85th Legislative Session “Pave it Forward” summary of changes

impacting TxDOT

https://bookstore.transportation.org/item_details.aspx?ID=114

https://www.roadsbridges.com/tti-study-hov-lanes-without-positive-separation-linked-car-crashes

http://www.statutes.legis.state.tx.us/Docs/TN/htm/TN.228.htm

https://policy.tti.tamu.edu/wp-content/uploads/2017/09/85th-Legislature-Summary-Pave-It-Forward-2017.pdf


HOV LANE GUIDANCE (APP. F)

47

• 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.


CALCULATION OF DESIGN HOUR VOLUME (DHV)

48

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 30th 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.


CALCULATION OF DESIGN HOUR VOLUME (DHV)

49

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.


THANK YOU !

Kenneth Mora, P.E. (Design Division)

10/10/2017

Documents

What’s in the roadway design manual - Texas A&M University · THE FUTURE OF THE TxDOT ROADWAY DESIGN MANUAL 2017 Short Course ... TxDOT updates to Roadway Design Manual Incorporating