Roadway Design Manual: Roadway Design Criteria

Departure from these guidelines are governed in Design Exceptions, Design Waivers, Design Variances, and Texas Highway Freight Network (THFN) Design Deviations, Chapter 1.

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Lane Width and Number

The usual and minimum lane width is 13-ft. The number of lanes required to accommodate the anticipated design year traffic is determined by the level of service evaluation as discussed in TRB’s Highway Capacity Manual.

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Shoulders

The minimum shoulder width is 12-ft. This width applies to both inside and outside shoulders, regardless of the number of main lanes. Shoulders must be continuously surfaced and be maintained.

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Pavement Cross Slope

Multilane divided pavements must be inclined in the same direction. The recommended pavement cross slope is 2 percent. Shoulders should be sloped sufficiently to drain surface water but not to an extent that safety concerns are created for vehicular use. To facilitate pavement drainage, highways with three or more lanes inclined in the same direction should have an increasing cross slope as the distance from the crown line increases. In these cases, the first two lanes adjacent to the crown line may be sloped flatter than normal-typically at 1.5 percent but not less than 1 percent. The cross slope of each successive pair of lanes (or single lane if it is the outside lane) outward from the crown should be increased by 0.5 to 1 percent from the cross slope of the adjacent lane. A cross slope should not exceed 4 percent on a tangent alignment unless there are three or more lanes in one direction of travel. Bridge structures with three or more lanes in one direction should maintain a constant slope of 2.5 percent, transitioning before and after the bridge accordingly.

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Vertical Clearances at Structures

The minimum vertical clearances at structures are shown in Table 2-11.

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Stopping Sight Distance

Stopping sight distance (SSD) for these facilities is calculated using the same methodology described in Chapter 2, Section 3, Sight Distance. The key variables that affect the calculation of SSD are brake reaction time and deceleration rate. The calculated and design stopping sight distances are shown in Table 8-1. Significant downgrades may affect stopping sight distances.

Anchor: #i1017354Table 8-1: Stopping Sight Distances for 5R Projects
Design Speed (mph)	Brake reaction distance (ft)	Braking distance on level (ft)	Stopping Sight Distance
Design Speed (mph)	Brake reaction distance (ft)	Braking distance on level (ft)	Calculated (ft)	Design (ft)
85	313.5	693.5	1,007	1,010
90	330.8	777.5	1,108.2	1,110
95	349.1	866.2	1,215.4	1,220
100	367.5	959.8	1,327.3	1,330
Note: Brake reaction distance predicated on a time of 2.5-sec; deceleration rate 11.2-ft/sec.

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Grades

Undesirable speed differentials between vehicle types suggest that limiting the rate and length of the grades be considered. Passenger vehicles are not significantly affected by grades as steep as 3 percent, regardless of initial speed. Grades above 2 percent may affect truck traffic depending on length of grade.

Table 8-2 summarizes the maximum grade controls in terms of design speed.

Anchor: #i1017436Table 8-2: Maximum Grades for 5R Projects
Terrain	Design Speed (mph)
Terrain	85	90	95	100
Level	2-3	2-3	2-3	2-3
Rolling	4	4	4	4

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Horizontal Alignment

Table 8-3 shows the maximum allowable side friction factors and assumed running speeds for design speeds from 85-mph to 100-mph. The maximum side friction force is used for full superelevation in conditions where limited space places constraints on the horizontal geometry and should be avoided.

Anchor: #i1248161Table 8-3: Side Friction Factors and Running Speeds for Horizontal Curves
Design Speed (mph)	Maximum Allowable Friction Factor	Running Speed (mph)
85	0.07	67
90	0.06	70
95	0.05	75¹
100	0.04	82¹
Note: Values adjusted up to accommodate application of AASHTO Method 5 calculations.

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Superelevation

Table 8-4 and Table 8-5 show minimum superelevation rates of various radii and design speeds for an e_max of 6 percent and 8 percent, respectively. For multi-lane facilities, particularly where wide medians are used, the radius applies to the innermost travel lane.

Anchor: #i1255624Table 8-4: Minimum Radii and Superelevation Rates¹ for Mobility Corridors, e_max = 6%
Superelevation Rate, e (%)	Radius, R (ft)
Superelevation Rate, e (%)	85-mph	90-mph	95-mph	100-mph
NC^2,4	29,310	32,190	37,140	44,420
RC^3,4	14,290	15,850	18,350	22,010
2.2	12,930	14,360	16,650	19,970
2.4	11,790	13,120	15,230	18,270
2.6	10,830	12,070	14,020	16,830
2.8	10,000	11,170	12,990	15,600
3.0	9,290	10,400	12,100	14,530
3.2	8,660	9,710	11,320	13,590
3.4	8,110	9,110	10,630	12,770
3.6	7,610	8,580	10,010	12,040
3.8	7,170	8,100	9,460	11,380
4.0	6,770	7,660	8,970	10,790
4.2	6,410	7,270	8,520	10,250
4.4	6,080	6,920	8,110	9,770
4.6	5,780	6,590	7,740	9,330
4.8	5,510	6,300	7,400	8,920
5.0	5,260	6,020	7,090	8,540
5.2	5,020	5,770	6,800	8,200
5.4	4,790	5,530	6,530	7,880
5.6	4,550	5,310	6,280	7,580
5.8	4,260	5,040	6,020	7,280
6.0	3,710	4,500	5,470	6,670
Notes: Computed using Superelevation Distribution Method 5. See AASHTO’s A Policy on Geometric Design of Highways and Streets for the different types of Superelevation Distribution Methods. a) The term “NC” (normal crown) represents an equal downward cross-slope, typically 2%, on each side of the axis of rotation. b) The minimum curve radii for normal crown are suitable up to 3.0%. c) 3.0% normal crown should only be used when 3 or more lanes are sloped in the same direction. d) 1.5% or flatter normal crown should only be used for the design of special circumstance, such as table-topping intersections, or the evaluation of existing conditions. The term “RC” (reverse crown) represents a curve where the downward, or adverse, cross-slope should be removed by superelevating the entire roadway at the normal cross-slope rate. For curve radii falling between normal crown and reverse crown, a superelevation rate equal to the normal crown should typically be used.

Anchor: #i1257009Table 8-5: Minimum Radii and Superelevation Rates¹ for Mobility Corridors, e_max = 8%
Superelevation Rate, e (%)	Radius, R (ft)
Superelevation Rate, e (%)	85-mph	90-mph	95-mph	100-mph
NC^2,4	29,700	32,580	37,530	44,870
RC^3,4	14,700	16,220	18,730	22,400
2.2	13,330	14,740	17,020	20,360
2.4	12,200	13,500	15,600	18,660
2.6	11,240	12,450	14,400	17,220
2.8	10,420	11,550	13,370	15,990
3.0	9,700	10,780	12,470	14,920
3.2	9,080	10,100	11,690	13,990
3.4	8,530	9,490	11,000	13,160
3.6	8,040	8,960	10,390	12,430
3.8	7,600	8,480	9,840	11,770
4.0	7,210	8,050	9,350	11,180
4.2	6,850	7,660	8,900	10,650
4.4	6,530	7,310	8,490	10,160
4.6	6,230	6,990	8,120	9,720
4.8	5,960	6,690	7,780	9,320
5.0	5,710	6,420	7,470	8,940
5.2	5,480	6,170	7,180	8,600
5.4	5,260	5,930	6,910	8,280
5.6	5,060	5,720	6,670	7,980
5.8	4,880	5,520	6,440	7,700
6.0	4,710	5,330	6,220	7,450
6.2	4,550	5,150	6,020	7,210
6.4	4,390	4,990	5,830	6,980
6.6	4,250	4,830	5,650	6,770
6.8	4,120	4,690	5,490	6,570
7.0	3,990	4,550	5,330	6,380
7.2	3,870	4,420	5,180	6,200
7.4	3,760	4,300	5,040	6,030
7.6	3,640	4,180	4,900	5,870
7.8	3,510	4,070	4,780	5,720
8.0	3,210	3,860	4,630	5,560
Notes: Computed using Superelevation Distribution Method 5. See AASHTO’s A Policy on Geometric Design of Highways and Streets for the different types of Superelevation Distribution Methods. a) The term “NC” (normal crown) represents an equal downward cross-slope, typically 2%, on each side of the axis of rotation. b) The minimum curve radii for normal crown are suitable up to 3.0%. c) 3.0% normal crown should only be used when 3 or more lanes are sloped in the same direction. d) 1.5% or flatter normal crown should only be used for the design of special circumstance, such as table-topping intersections, or the evaluation of existing conditions. The term “RC” (reverse crown) represents a curve where the downward, or adverse, cross-slope should be removed by superelevating the entire roadway at the normal cross slope-rate. For curve radii falling between normal crown and reverse crown, a superelevation rate equal to the normal crown should typically be used.

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Superelevation Transition

Desirable design values for length of superelevation transition are based on a given maximum relative gradient between profiles of the edge of traveled way and the axis of rotation. Table 8-6 shows recommended maximum relative gradient values. Transition length on this basis is directly proportional to the total superelevation, which is the product of the lane width and the change in the cross slope. For superelevation on bridge structures, it is preferred to begin/end superelevation transition at a bridge bent line.

Anchor: #i1250304Table 8-6: Maximum Relative Gradient for Superelevation Transition
Design Speed (mph)	Maximum Relative Gradient¹ (%)	Equivalent Maximum Relative Slope (V:H)
85-100	0.50	1:200
Note: Maximum relative gradient for profile between edge of traveled way and axis of rotation.

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Vertical Curves

Vertical curves create a gradual transition between different grades which is essential for the safe and efficient operation of a roadway. The lengths of both crest and sag vertical curves are controlled by the available sight distance. Vertical curves are required for all grade breaks.

Minimum K-values are calculated using the same equations as in Chapter 2, Section 6.

Design Ks for both crest and sag vertical curves are shown on Table 8-7.

Anchor: #i1018132Table 8-7: Minimum Design K for Crest and Sag Vertical Curves
Design Speed (mph)	Stopping Sight Distance (ft)	Crest Vertical Curves (K)	Sag Vertical Curves (K)
85	1,010	473	260
90	1,110	571	288
95	1,220	690	319
100	1,330	820	350

The length of a sag vertical curve that satisfies the driver comfort criteria is 60 percent of the sag vertical curve length required by the sight distance control. Driver comfort control should be reserved for special use and where continuous lighting systems are in place.