§ 1.3.1. General Design Criteria

A.

Grades. The following design criteria is based on material from the Institute of Transportation Engineers Report, Guidelines for Urban Major Street Design, 1983, Sections 6.1, 6.3 and 6.4.

Grades have an economic effect on vehicle operating costs and time losses and they also affect highway capacity and safety.

The grade line is a series of straight lines connected by parabolic vertical curves to which the straight lines are tangent. Under all conditions this line should be smooth flowing. Short, choppy grades are unsightly and disrupt operating conditions.

1.

Maximum Grades.

Maximum grades are determined primarily by the operation characteristics of vehicles on grades. Driving practices with respect to grades vary greatly, but nearly all passenger cars can readily negotiate upgrades as steep as seven (7) to eight (8) percent. Passenger vehicle speeds decrease progressively with steeper grades.

The effect of grades on bus or truck speeds is most pronounced. On upgrades, the maximum speed a bus or truck can maintain is dependent on the grade length and steepness, and on the ratio of the gross vehicle weight to engine horsepower. This will not only affect speed, but may also be a pronounced effect on the capacity of the street where there are appreciable bus and/or truck volumes. Tables 1-7, 1-8 and 1-12 indicate maximum permissible grades. However, such grades should be used infrequently.

The maximum gradient range for roadways carrying bus traffic is six (6) to eight (8) percent. ( Urban Public Transportation: Systems and Technology , Vukan R. Vuchic. Englewood Cliffs, NJ: Prentice-Hall, 1981). To adequately exploit the travel time and speed advantage of an exclusive bus lane, the maximum recommended grade for a high-occupancy vehicle (HOV) lane or busway is four (4) percent (Institute of Transportation Engineers, Transportation and Traffic Engineering Handbook , 2nd ed. Englewood Cliffs, NJ: Prentice-Hall, 1982).

2.

Minimum Grades.

Minimum grades are governed by drainage conditions. With curbed pavements, longitudinal grades should be provided to facilitate surface drainage. A minimum grade of 0.4 percent is used.

3.

General Controls for Vertical Alignment.

The following are general design controls which should be addressed in determining vertical alignments:

• The grade line should be smooth flowing.

• The "roller coaster" type profile should be avoided.

• Undulating grade lines should be appraised for their effect upon traffic operations.

• A broken-back grade line (successive vertical curves in the same direction) generally should be avoided.

• It is desirable to reduce the grade through intersections on roadways with moderate to steep grades.

• A sag vertical or flat grade is desirable in advance of such features as channelizations and ramp takeoffs in order to provide good visibility.

• Steep downgrades should be avoided, whenever practicable, at the approach to traffic signals and stop signs.

4.

Vertical Curves.

Vertical curves should be simple in application and should result in a design that is safe, comfortable in operation, pleasing in appearance and adequate for drainage.

For simplicity, the parabolic curve with an equivalent axis center on the vertical point of intersection is recommended in roadway profile design (see Figure 1-3 contained in Appendix H of this manual).

Figures 1-4 and 1-5 contained in Appendix H of this manual indicate the length of vertical curve in relation to algebraic difference in grades necessary to maintain safety and comfort for crest vertical curves and sag vertical curves.

Maximum grade breaks of 0.8 percent or less may be used without a vertical curve.

Note that sight distance required from intersecting streets or driveways along vertical curves is not addressed in Figures 1-3, 1-4 and 1-5 contained in Appendix H of this manual. An intersecting street or driveway may not be appropriate along a vertical curve when required sight distance from side street or driveway is not attainable. If it is essential that a side street or driveway intersect the main street along a vertical curve, then it may be necessary to reduce the vertical curve so that necessary sight distance is available. Horizontal and vertical alignments should not be designed independently. They complement each other and poorly designed combinations can spoil the good points and aggravate the deficiencies of each. Horizontal alignment and profile are among the more important design elements of a roadway.

Comfort control criteria as defined in AASHTO for sag vertical curves is generally discouraged. This criteria, however, may be used when the subdivision is provided with adequate fixed-source lighting (street lights). The Director of the Watershed Protection and Development Review Department shall be consulted in regard to procedural and funding requirements pertaining to fixed-source lighting.

B.

Minimum Horizontal Radii.

The following design criteria is based on material from the American Association of State Highway and Transportation Officials (AASHTO) Manual, A Policy on Geometric Design of Highways and Streets , 1984, Chapters III, V, VI and VII.

The minimum radius of a roadway is directly related to a roadway's design speed, superelevation and side friction factor.

The 1984 AASHTO Manual, Figure III-17, "Maximum Safe and Comfortable Speed for Horizontal Curves on Low-speed Urban Streets," was utilized in establishing the following radii:

For a superelevation (e) = -0.02, typical for normal crown

A design speed of:

20 mph relates to a minimum allowable radius of 100 ft.

25 mph relates to a minimum allowable radius of 180 ft.

30 mph relates to a minimum allowable radius of 300 ft.

35 mph relates to a minimum allowable radius of 470 ft.

40 mph relates to a minimum allowable radius of 725 ft.

From 1984 AASHTO Manual, Figure III-7, "Side Friction Factors for Rural Highways and High-speed Urban Streets,"

A design speed of:

45 mph relates to a side friction factor (f) of 0.145

50 mph relates to a side friction factor (f) of 0.140

The minimum safe radius (R) is calculated from the formula

R=V /[15(e+f)] (eq. 1-1)

For design speed (V) = 45 mph,

R=(45 )/[15(-0.02+0.145)] = 1,080, say 1,000 feet

For design speed (V) = 50 mph,

R=(50 )/[15(-0.02+0.140)] = 1,389, say 1,400 feet

The above values for the minimum horizontal radii are reflective of the standards set forth in this document.

Superelevation rate, "e" may be varied thereby resulting in different values for "R" (minimum centerline radius). Changes in the above values for "e", however, should lend consideration to intersecting cross street designs. Tangent lengths between curves may also need to be extended to provide for proper superelevation runoff.

C.

Cross Slope.

The following design criteria were adapted from the American Association of State Highway and Transportation Officials (AASHTO) Manual, A Policy of Geometric Design of Highways and Streets , 2001, Chapter IV.

On two-lane roadways that are crowned at the center, the accepted rate of cross slope ranges from 1.5% to 2%. When three or more lanes are inclined in the same direction on multilane roads, each successive pair of lanes or portions thereof that are positioned away from the first two lanes at the crown may have an increased slope. The two lanes adjacent to the crown line should be pitched at the normal minimum slope, and on each successive pair of lanes or portion thereof outward, the rate may be increased by 0.5% to 1.0%. Therefore, the minimum lane cross slope shall be 1.5%, while the typical lane cross slope shall be 2%. The maximum cross slope for outside lanes of multilane roads or streets shall be 3%.

Use of cross slopes greater than 2% on roads or streets with a central crown line is not desirable. In passing maneuvers, drivers cross and recross the crown line and negotiate a total rollover or cross slope change of over 4%. The reverse curve path of the passing vehicle causes a reversal in the direction of centrifugal force, which is further exaggerated by the effect of the reversing cross slopes. Trucks with high centers of gravity crossing over the crown line are caused to sway from side to side, which at times may be difficult to control. Therefore, the maximum algebraic difference between cross slopes in adjacent lanes, main lanes and auxiliary lanes shall be 4% for a crown or crest slope break.

On roadway section that incorporates median islands, the difference in curb heights between the two interior curb lines may vary. In the area of intersections, median openings or possible median openings, the slope between the two interior curb tops should be no more than 2%. Designs which utilize greater slopes will be individually reviewed by TPSD staff. Waivers may be approved for locations where drainage, landfill or environmental issues may require greater slope. Median openings are to be held to the same standard as intersections because driveway connections may be made to produce a three or four-legged intersection.

D.

Intersection Design.

The following design criteria was adapted from the Institute of Transportation Engineers reports, Recommended Guidelines for Subdivision Streets , Section 2.05 and Guidelines for Major Street Design , Section 9.5. This section lends guidance for proper intersection design regarding proper roadway alignments in the intersection area.

1.

Vertical Alignment within Intersection Area.

Intersection areas should be designed with a flat grade. In the more difficult terrains, this becomes economically impractical.

The design speed for the major street at any intersection shall be maintained through the intersection approaches. The minor street may be designed with a change in grade based on reduced design speeds between the maximum grade in the approach and the cross-slope of the intersected street not to exceed eight (8) percent for local streets and six (6) percent for collector streets. The change in grade shall be accomplished by means of a vertical curve of length equal to the minimum length for that approach for that intersection type as indicated in Tables 1-7 and 1-12.

2.

Horizontal Alignment within Intersection Area.

The horizontal approach to an intersection should be tangent for a length of 50-100 feet (see Tables 1-7 and 1-12). Note that these tangent lengths are minimum. Longer tangents are highly desirable. The tangent distance is measured from the curb line of one (1) street to the first point of curvature on the intersecting street. In this regard, radii greater or equal to 1000 feet may be considered tangent.

Where driveways are not limited to right in and right out movements, requirements for local streets (as indicated in Tables 1-7 and 1-12) should apply. It is desirable for all intersections to meet at approximately a 90 degree angle. However, necessary sight distance for streets intersecting from the outside of a curve is generally attainable. Skewed intersections should be avoided and in no case should the angle be less than 80 degrees or greater than 100 degrees. Studies have shown that skewed intersections have generally higher accident rates than those intersecting at 90 degrees. Desirable alignments will also provide for increased visibility of traffic control devices such as stop signs or yield signs and will also provide increased visibility of cross traffic.

3.

Minimum Curb Radius.

As curb radius is increased, paving costs and intersection area required for a pedestrian to traverse are increased and higher turning speeds are encouraged. Substandard radii result in unnecessary lane encroachment and increased traffic conflict and accident potential. Reasonable design values of 15 feet are recommended for intersection radii of two (2) local streets, based on curb clearance of three (3) feet and without lane encroachment for a typical width street, using the AASHTO design passenger vehicle. This design will also accommodate garbage trucks and moving vans with wide swings. An increased radius of 20 feet for the local-collector or collector-collector intersection is predicated upon a desire to slightly improve the maneuverability of a vehicle in entering or leaving the collector. A collector intersection with an arterial street should have a 25 foot radius. An arterial-arterial intersection should have a 30 foot radius.

4.

Minimum Centerline Offset of Adjacent Intersection.

Several studies of intersection design types have shown T-type intersections to be far safer than cross-type. Extensive use of T intersections in residential subdivisions is strongly recommended. One disadvantage, however, is "corner cutting" when inadequate offset exists between adjacent intersections. To reduce this hazardous practice, offsets of at least 150 feet between center lines are required. In the case of two (2) collector-street intersections, this offset shall not be less than 300 feet in order to allow for left-turn storage between intersections.

Offset intersections have disadvantages when one (1) or both such streets is a collector intersecting an arterial street, if volumes will be such to warrant traffic signals. Operations at such locations are more complicated than those for normal cross-type intersections. Therefore, other design solutions should be sought if signalization might otherwise be required. When offset intersections are used at an arterial street, they should be located to avoid conflicting left turns (this is especially important where two (2) way, left-turn lanes are to be provided or where left-turn slots are used in a fairly narrow median). Such left-turn conflicts exist when an intersection offsets to the right rather than to the left.

The distance between intersection offsets is measured from the center line intersection of one (1) intersecting roadway and the centerline intersection of the next intersecting roadway, measured along the centerline of the intersected roadway. Multileg intersections [over four (4)] are undesirable from the control and safety standpoint.

5.

Drainage Structures.

The location of drainage structures, inlets, catch basins, etc., should be consistent with the intended use of the roadway and in accordance with the Drainage Criteria Manual .

Inlets or catch basins should not be located within the corner curb return or within ten (10) feet from the point of curvature of the curb return. Clearance is needed to allow space for street lights, street name signs, utility poles, pedestrians, sidewalk ramps, etc.

At intersections which have valley drainage, the crowns of the intersecting streets will culminate in a distance of 40 feet from the intersecting curb lines unless otherwise noted on the construction plans. Inlets on intersecting streets shall not be constructed within 50 feet of the valley drainage.

Valley gutters should not be designed across streets with collector or higher classification.

6.

Sight Distance.

Intersections should be planned and located to provide as much sight distance as possible. A basic requirement for all controlled intersections is that drivers must be able to see the control device well in advance of performing the required action. Stopping sight distance on all approaches is needed as a minimum. Obstruction-free sight triangles are desirable, in both the horizontal and vertical planes as related to assumed driver eye height and position.

The stopping sight distance (SSD) in feet is determined from the formula:

Where

V = Design speed in miles per hour

P = Perception-reaction time in seconds (2.5 seconds)

f = Coefficient of friction for wet pavement (.30 to .40)

g = Percent of grade divided by 100 (+ for upgrade; - for downgrade)

Entering intersection sight distance (ISD) at intersections controlled by "stop" signs may be measured as shown in Figure 1-6 contained in Appendix H of this manual. The resultant sight triangle should be free of sight obstructions such as parked vehicles, buildings, walls, hedges, bushes, low growing trees or guardrail (if located on a crest where the rail forms a sight restriction), above an assumed driver eye height line of sight to target.

This height is three and one-half (3.5) feet for passenger cars and six (6) feet for SU and WB-50 design vehicles, related to an approaching vehicle (target) four and one-fourth (4.25) feet high. The sight line is based on the time required for the stopped vehicle to clear the intersection versus the distance a vehicle will travel along the major street. See Table 1-1 for minimum sight distance.

7.

Median Design at Intersections.

End treatment of medians at intersections should be designed to accommodate the design vehicle turning at a reasonable rate of speed. Semicircular radii may be used on the noses of medians up to six (6) feet wide. Bullet-nosed medians should be used for medians of greater width. A minimum 50 foot control radius or a 75 foot control radius is required as stated in Table 1-2. Figures 1-7 and 1-8 contained in Appendix H of this manual illustrate examples of providing adequate curves at the medians.

TABLE 1-1 MINIMUM SIGHT DISTANCE* Design Speed for Major Roadway (mph)
Case	Condition	30	35	40	45	50	55
I	A	200	250	325	400	475	550
	B**	350	410	470	530	590	650
	C**	400	460	520	580	640	700
II	A	400	470	540	610	680	750
	B**	475	550	625	700	775	850
	C**	510	590	670	750	830	900
III	A	400	500	650	800	1000	1200
	B**	475	550	650	800	1000	1200
	C**	510	590	670	800	1000	1200
Where, CASE I is the Absolute minimum from driveways (other than industrial driveways).
CASE II is the Absolute minimum from all streets and industrial driveways.
CASE III is the Desirable minimum from all driveways and all streets.
Condition A - Entering onto or crossing a 2 or 3 lane street.**
Condition B - Entering onto or crossing a 4 or 5 lane street.***
Condition C - Entering onto or crossing a street with more than 5 lanes.***
* This Table is for flat grades only. AASHTO should be referenced in obtaining minimum sight distances whenever grades are not flat.
** For each Case Type, minimum sight distances to the left need not exceed distances identified for Condition A.
*** If median shadowing is > 20′, then the intersection can be considered as 2 separate intersections, utilizing Condition A.
Source: City of Austin, Department of Public Works and Transportation

 

TABLE 1-2 REQUIRED CONTROL RADIUS
STREET CLASSIFICATIONS
MAJOR STREET	MINOR STREET	CONTROL RADIUS
Major Arterial	Major Arterial	75′
Major Arterial	Minor Arterial	75′
Major Arterial	Primary Coll., Divided	75′
Major Arterial	Collectors, Undivided	50′
Major Arterial	Local Streets*	50′
Minor Arterial, Divided	Minor Arterial	75′
Minor Arterial, Divided	Primary Coll., Divided	75′
Minor Arterial, Divided	Collectors, Undivided	50′
Minor Arterial, Divided	Local Streets*	50′
Minor Arterial, Undivided	Primary Coll., Divided	75′
Primary Coll., Divided	Primary Coll., Divided	50′
Primary Coll., Divided	Collectors, Undivided	50′
Primary Coll., Divided	Local Streets	50′
* Local streets should not generally be designed to intersect with arterial streets.
Source: City of Austin, Department of Public Works and Transportation

 

E.

Tapers.

The following design criteria was adapted from the Institute of Transportation Engineers report, Guidelines for Urban Major Street Design , 1983, Sections 4.1 through 4.8 and the Uniform Design Standards Manual of the Metropolitan Transit Authority of Harris County.

1.

Terminology.

In order to discuss the various elements of turn-lane channelization, a standard terminology must be established. Figure 1-9 contained in Appendix H of this manual illustrates the concepts.

• APPROACH TAPER is that portion of the roadway geometry from the point where all approaching traffic must shift laterally, to the beginning point of the bay taper.

• BAY TAPER is from the edge of the adjacent through traffic lane to the beginning of the full width of the turn storage lane.

• STORAGE LENGTH is the distance from the end of the bay taper to the intersection nose or stop line.

• INTERSECTION NOSE is the radius or distance from the end of the storage bay to the near edge of the cross-route exit lane for the left-turning vehicle. For left-turn bays the cross-route exit reference is normally the centerline of an unchannelized two (2) way street or the far edge of the median in a channelized street.

• DEPARTURE TAPER of a left-turn bay is from the point where through traffic beyond the intersection begins a lateral shift to the left to the point where the through lane is adjacent and parallel to the centerline.

2.

Approach Tapers.

In general, a taper that causes all vehicles to transition laterally should be moderately long. However, lengths are constrained in urban areas. The taper length is a direct product of slope angle, which is most related to expected operating speeds.

In a study by the Department of Public Works, County of Sacramento, California, three (3) equations were tested to establish taper lengths.

L = W*S (eq. 1-3)

L = (W*S*S)/60 (eq. 1-4)

Where

L = Length in feet

S = Design Speed - Speed in miles per hour

W = Lateral offset in feet from centerline (double yellow)

This study included recording the speed of vehicles at the beginning and end of the approach taper and noting where the driver stays between the lane lines. Based on the results:

Equation 1-4 is recommended for posted speeds of 40 mph or less.

Equation 1-3 is recommended for posted speeds of 45 mph or greater.

See Figure 1-9 and Figure 1-10 in Appendix H of this manual for approach tapers.

3.

Bay Tapers.

There is a great disparity throughout the United States in the design of bay tapers. The transition lengths and the corresponding curve radii vary widely because of the different philosophies regarding the type of entry a turning vehicle should make. Some feel that the transition should be smooth and gradual. Thus, in these areas, large radii and long transition lengths are used. This type of design also is favored because of the ease of street cleaning operations.

Opponents of this philosophy point out that unobservant drivers will tend to drift into the turn lane and may cause unnecessary conflicts with other vehicles as they try to get back to the through lane. It is their contention that to avoid this possible conflict, turn lanes should stand out and that short transition lengths and small radii should be used. The use of shorter bay tapers also increases the potential storage length for closely spaced intersections or major driveways.

The design of bay tapers in the City of Austin should conform to standards indicated in Figure 1-11 contained in Appendix H of this manual.

4.

Departure Tapers for Left-Turn Bays.

There are two (2) different designs for developing the departure taper. The variation relates to the point of the start of the taper in a channelization that provides a full shadowed lane. The first variation starts the taper at the point of full median width, while the other begins the taper at the end of the storage lane, as shown in Figure 1-10 contained in Appendix H of this manual. Beginning at the end of the storage lane and ending at the beginning of the approach taper provides a flatter angle which is easier for a vehicle to negotiate. It requires less widening and/or parking restrictions and is recommended as the desirable design guide.

5.

Acceleration/Deceleration Lanes.

Acceleration lanes are seldom used along urban major streets. However, when they are used, the transition taper design may be the same as for an approach taper. Many agencies utilize the rule of thumb that allows one (1) foot of lateral displacement per mph of the roadway into which the vehicle is emerging (L=W*S). Thus, a 12 foot (W) acceleration lane merging with a street having a speed of 30 mph (S) would produce a 360 foot taper. A deceleration lane is actually a right-turn lane (or left if on a one (1) way roadway) and therefore should be designed in accordance with bay-taper principles.

6.

Through-lane Tapers.

When all traffic must transition to the left or right, the design represents an approach-taper condition. For an added through lane approaching an intersection, the transition into the lane may be made by either an approach or a bay-taper design. However, the termination of the added lane beyond the intersection (a lane drop) should be handled by the approach-taper type of design.

7.

Tapers on Horizontal Curves.

For a left-turn bay, the taper may be longer if the horizontal curve direction is to the driver's left. Conversely, the tapers may be shorter if the curve is to the driver's right. Figure 1-12 contained in Appendix H of this manual illustrates the reason for this, based upon the deflection angle required from a tangent line to the curve.

Adjustment of "standard" local design criteria is most appropriate for turn-lane tapers located on curves of 500 foot radius or less. The adjustment can be determined graphically.

F.

Median and Median Breaks.

The following design criteria was adapted from the Institute of Transportation Engineers report, Guidelines for Urban Major Street Design , 1983, Sections 7.1 through 7.4. and Section 12.5.

1.

Function.

A median is that portion of a divided highway separating the traveled way for traffic in opposite directions. Medians or two (2) way left turn lanes (2-WLTL) should be considered for all major urban streets of four (4) or more lanes.

Medians can provide major benefits to traffic operations on the route. Curbed medians can provide space for traffic control devices, for storage of left-turn and/or U-turn traffic. Flush medians can also provide a recovery area for out-of-control vehicles and an emergency stopping place for disabled vehicles. Some median designs can reduce headlight glare and serve as a refuge area for pedestrians and bicyclists. Where sufficient width is provided, medians may allow for future expansion of the through roadway. Good median design can smooth traffic flow and reduce conflicts.

Other functional and urban design values may be enhanced by medians. Wider medians can provide a location for drainage systems, lighting, utilities and other roadway facilities. A well-designed median can lend an orderly and attractive character both to the neighborhood and to the street that it serves.

An eight (8) to 20 foot curbed median often represents a good trade-off of operational advantages and disadvantages, if used for major streets on new alignment or through undeveloped areas and where access limitations are practical (reverse lot frontage subdivisions or combined access for direct connection tracts). With crossover access spacings of 600 to 1,000 feet, including left-turn bays, a balance can be struck between efficient service to through traffic and secondary service to abutting development.

Wider roadways requiring three (3) lanes in each direction, resulting in seven (7) lanes if used-with the 2-WLTL concept, can produce severe problems for pedestrian crossings. The unprotected width to be traversed can be unsafe at local or mid-block locations and can restrict traffic signal efficiency. Where such large numbers of lanes are needed, curbed medians may be warranted.

2.

Types.

Medians may be depressed, raised or flush with respect to their adjacent traveled way. Depressed medians may be edged with raised curbs or they may slope from the edge of the roadway directly. Often sections wider than 16 feet are depressed to collect drainage. Side slopes of 10:1 (6:1 minimum) are preferred to allow for vehicle recovery. Flush medians are typically narrow and paved. They do not prevent access to adjacent property and serve the purpose of separating opposing flows at less cost. Raised medians may be preferred for access control and landscaping purposes where drainage is not a problem. Raised medians also provide a positive visual barrier which prevents erratic cross-traffic movements.

3.

Median Width.

The width of a median is its most important geometric design consideration. Table 1-3 indicates widths necessary to accomplish certain functions, based on the passenger vehicle for primary design of crossing protection and U-turns (see Figures 1-13 through 1-18 in Appendix H of this manual for various median applications).

TABLE 1-3 RECOMMENDED MEDIAN WIDTHS (FOC TO FOC)
Function	Minimum (feet)	Desirable (feet)
Separation of Opposing Traffic	4*	6*
Pedestrian Refuge and Space for Traffic Control	6*	16
Left-Turn, Speed-change and Storage	14	16
Crossing/Entering Vehicle Protection	20	23
"U"-Turns, speed change and storage	20	23
Channelized "T", speed-change and storage	20	23 - 30
* Cannot accommodate left-turn lanes, hence, such turns must be made from the through lanes.
Source: City of Austin, Department of Public Works and Transportation Based on ITE, Guidelines for Urban Major Street Design

 

4.

Median Break Spacing.

The fewer driveways on a major, urban street, the more effectively it will serve its primary function. Spacings should be maintained between driveways and intersections appropriate to the character of the driveway and roadway.

Driveway spacing should allow reasonable deceleration of vehicles approaching on the street and acceleration by vehicles entering the street. Median breaks for driveways should not be contemplated unless sufficient length is available to accommodate deceleration tapers and storage lengths. Table 1-4 reflects median and median break criteria. This criteria is based on the National Cooperative Highway Research Program (NCHRP) Report No. 93.

Full-function median openings (Figures 1-19, 1-20 and 1-21 in Appendix H of this manual) on major arterials should be allowed only where the minimum spacings for signalized intersections are practicable. At intermediate locations along major arterials, limited-function openings may be provided at the spacings listed in Table 1-4.

High volume driveways on major arterials should only be located opposite streets or other driveways when the minimum spacing requirements for signalized locations are met. Otherwise, T-intersection configurations should be designed. When driveways are located opposite street intersections the two (2) should have compatible design elements.

On streets other than major arterials, full-function median openings are acceptable at the spacings listed in Table 1-4. On both major and minor arterials, access to public streets will have priority over access to private property.

G.

Turn Lanes and Channelization.

The following design criteria was adapted from the ITE Report, Guidelines for Urban Major Street Design , 1983, Sections 9.3 and 9.4. and from the AASHTO manual, A Policy on Geometric Design of Highways and Streets , 1984 and from Research Report 258-1, Project 3-18-80-258 , Center for Transportation Research, Bureau of Engineering Research, The University of Texas at Austin, January, 1984.

1.

Turn Lanes.

The primary purpose of left-turn lanes at intersections is to provide storage space. A secondary purpose of turn lanes is to provide a location for deceleration removed from the through traffic lanes, thereby maintaining the capacity of the through roadway. Studies have demonstrated that accident experience is significantly reduced when left-turn lanes are provided at intersections of two (2) major streets, i.e., collectors and arterials.

Table 1-4 Median Opening Criteria
Design Speed (mph)	Minimum Spacing* Distance "C" from Figure 2-19		Minimum Spacing Distance B+C from Figure 1-19
			100′ Min. Storage Requirement**		150′ Min. Storage Requirement***
	Absolute (ft)	Desirable (ft)	Absolute (ft)	Desirable (ft)	Absolute (ft)	Desirable (ft)
30	200	350	300	450	350	500
35	250	425	350	525	400	575
40	300	500	400	600	450	650
45	350	600	450	700	500	750
50	450	750	550	850	600	900
* Plus storage lengths based on peak hour volumes (see Table 1-5)
** Minimum storage when turning into a local street
*** Minimum storage when turning into a collector or an arterial street
Source: Based on NCHRP Report No. 93, 1970

 

At a minimum, storage lengths should be 150 feet when turning into a collector or an arterial and 100 feet when turning into a local street. At any unsignalized intersections, the storage length, exclusive of taper may be based on the number of turning vehicles likely to arrive in an average two (2) minute period within the peak hour with each vehicle accounting for approximately 20 feet of storage. At signalized intersections, the storage length depends on the signal cycle length, the signal phasing arrangement and the rate of arrivals and departures of left-turning vehicles (see Table 1-5).

TABLE 1-5 STORAGE LENGTH OF LEFT TURN BAY (FOR ARTERIAL)**
Lmax(av)	City of Austin Standard	City of Austin Dual Left Standard
0	0	-
6	150	-
8	200	-
10	250	-
12	300	200
14	340	200
16	370	200
17	400	300
18	425	300
20	450	300
21	475	300
22	500	300
23	525	300
24	550	300
25	575	350
* Storage lengths exceeding 400 feet should be discouraged. All proposals for turn bays exceeding 400 feet will require the approval of the Director of the Department of Public Works and Transportation.
** Similar applications could be used for other street classifications.
Lmax (av) = 5.5 (Lavg 0.58 ) (based on average condition)
Source: Based on Research Report 258-1, University of Texas Center for Transportation Research, 1984

 

Dual left-turn and right-turn lanes are successful where traffic volumes exceed the capacity of a single lane and the cross-street is of sufficient width to receive two (2) vehicles turning abreast. For dual right-turn lanes and dual left-turn lanes from one (1) way streets, the inside lane must be a mandatory turn.

2.

Channelization.

Channelization of intersections is the separation or regulation of conflicting traffic movement into definite paths of travel by the use of pavement markings, raised islands or other suitable means to facilitate the safe and orderly movements of both vehicles and pedestrians. The main objectives of intersection channelization are to assure orderly traffic movement, increase capacity, improve safety and provide maximum convenience. To carry out these objectives, channelization is employed for one (1) or more of the following purposes:

• Separation of conflicts.

• Control of angle of conflict.

• Reduction of excessive pavement areas.

• Regulation of traffic and indication of proper use of the intersection.

• Arrangements to favor predominant turning movements.

• Protection of pedestrians.

• Protection and storage of turning and crossing vehicles.

• Location of traffic control devices.

• Prohibition of specific movements.

Small, isolated channelization islands should be avoided. Islands should be readily visible and designs with numerous small islands should be discarded in favor of those with a few large ones. Long narrow islands may be undesirable adjacent to turn lanes. Islands with at least 100 square feet are desirable but, under very restricted conditions, 75 square feet may be used. Islands used for pedestrian refuge desirably should be six (6) feet wide, with a minimum of four (4) feet. If wheelchair access is to be considered, the minimum width of a curb ramp shall be 36 inches, exclusive of flared sides.

The following principles should be considered and addressed in meeting conditions at particular intersections. However, if they are disregarded, the objectives of channeliza-tion will not be achieved and the resulting design may be hazardous and inefficient.

• Reduce the area of conflict; large paved intersectional areas invite hazardous vehicle and pedestrian movements.

• When traffic streams cross without merging and weaving, make the crossing at or near right angles. If traffic signal control is planned, the crossing angle may be less than right angle with suitable signal design and visual clues.

• Merge traffic streams at small angles.

• The speed of a traffic stream entering an intersection may be controlled by funneling.

• Provide refuge (shadowing) for turning and crossing vehicles where possible and necessary with channelization.

• Use channelization to separate conflict points within an intersection.

• Block prohibited turns with well-delineated channelization.

• Channelization may provide locations for the installation of essential traffic control devices to enhance their visibility.

H.

Environmental Considerations.

Application of the street design criteria contained in this document to new subdivisions and site developments must take into consideration all applicable environmental standards, including restrictions on cut and fill and development setbacks from waterways and critical environmental features. Requirements of the street design criteria shall not be considered as sole justification for variances from the Comprehensive Watersheds Ordinance or any of the Special Watershed Ordinances. Conversely, requirements of the Comprehensive Watersheds Ordinance or any of the Special Watershed Ordinances shall not be considered as sole justification for variances from street design criteria. It is advisable to delineate all required setbacks and other applicable environmental protection measures prior to designing streets.

Minor deviations from the street design criteria may be applied for, on a case-by-case basis, in order to protect specific environmental features on severely constrained tracts provided that proposed deviations meet minimum safety standards and are approved by the Directors of the Public Works Department, the Transportation, Planning and Sustainability Department and the Watershed Protection and Development Review Department. General deviations may be pursued as stated in the Preface of this Manual.

In the event that differences occur, the resolution procedure provided for in Title 25, the Land Development Code, applies.