Chapter 17 - Emergency Escape Ramps
Publication 13M (DM-2)
CHAPTER 17
EMERGENCY ESCAPE RAMPS
17.0 INTRODUCTION
Where long, descending grades exist or where topographic and location controls require such grades on new
alignment, the design and construction of an emergency escape ramp at an appropriate location is desirable to
provide a location for out-of-control vehicles, particularly trucks, to slow and stop away from the main traffic
stream. Out-of-control vehicles are generally the result of a driver losing braking ability either through overheating
of the brakes due to mechanical failure or failure to downshift at the appropriate time. The loss of stopping
capability of a heavy vehicle on a downgrade resulting in an "out-of-control" situation is a relatively infrequent
event. The results of that event, however, in many cases are spectacular and very costly in both lives lost and
property damage.
Even the best road design cannot fully compensate for the "out-of-control" problem in mountainous terrain, and
vehicle performance standards can provide for heavy vehicle control on long and/or steep downgrades only when
the use of gear shifting and braking are properly combined to reduce speeds.
Static signing and stopping areas (turnouts or pull off areas) located before severe downgrades, to be used for
voluntary or mandatory brake inspections and for cooling of brakes to restore their stopping capabilities, are the
methods most commonly used to provide proper information to the drivers and provide an opportunity for checking
the operation of the equipment prior to descent. In addition, information about the grade ahead and the location of
escape ramps can be provided by diagrammetric signing. Refer to Publication 212, Official Traffic Control Devices.
The Department has constructed and maintains emergency escape ramps throughout the Commonwealth. Reports
and evaluations indicate that these escape ramps have reduced property damage and more importantly have saved
lives. The design criteria presented in this chapter have been developed by the Department through research by The
Pennsylvania State University. The goal of this research project was to understand the physical characteristics of the
stopping mechanism and to provide a means for adequately designing and maintaining gravel arrester beds. Fullscale testing was performed at operational gravel arrester beds within the state as well as at two research gravel
arrester beds located at The Pennsylvania Transportation Institute's (PTI) Research Facility.
Additional information and details can be obtained from The Pennsylvania Transportation Institute, A Field and
Laboratory Study to Establish Truck Escape Ramp Design Methodology, Report No. FHWA-PA-86-034+83-23,
October 1988 and the AASHTO Green Book.
17.1 DYNAMICS OF A VEHICLE
The effectiveness of gravel arrester beds in stopping runaway vehicles results from the interaction between vehicle
motion and gravel movement. The motion can be predicted if the forces acting on the vehicle can be predicted
because Newton's law gives the deceleration if the forces and masses of the vehicle are known.
Resistance forces that act on every vehicle and affect the vehicle's speed include engine, braking and tractive
resistance forces. Engine and braking resistance forces can be ignored in the design of emergency escape ramps
because the ramp should be designed for the worst case; that is, the vehicle is out of gear and the brake system has
failed. Tractive resistance forces contain four subclasses: inertial, aerodynamic, rolling and gradient. Inertial and
negative gradient resistance forces act to maintain motion of the vehicle while rolling, positive gradient and air
resistance forces act to retard its motion. The 2004 AASHTO Green Book, Chapter 3, Exhibit 3-65 illustrates the
action of the various resistance forces on a vehicle in motion.
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Publication 13M (DM-2)
Inertial resistance force can be described as a force that resists movement in a vehicle at rest or keeps a vehicle in
motion, unless the vehicle is acted upon by some external force. Inertial resistance force must be overcome to either
increase or decrease the speed of a vehicle. Rolling and positive gradient resistance forces are available to overcome
the inertial resistance force. Rolling resistance force is a general term used to describe the resistance to motion at
the area of contact between a vehicle's tires and the roadway surface and is only applicable when a vehicle is in
motion. It is influenced by the type and displacement characteristics of the surfacing material of the roadway.
Gradient resistance force is due to the effect of gravity and is expressed as the force needed to move the vehicle
through a vertical distance. For gradient resistance force to provide a beneficial force on an escape ramp, the vehicle
must be moving upgrade against gravity. In the case where the vehicle is descending a grade, gradient resistance
force is negative, thereby reducing the forces available to slow and stop the vehicle. It is influenced by the total
mass (weight) of the vehicle and the magnitude of the grade.
The remaining component of tractive resistance force is aerodynamic resistance force. Air causes a significant
resistance at speeds above 80 km/h (50 mph) and is negligible under 30 km/h (20 mph). The effect of aerodynamic
resistance has been neglected in determining the length of the arrester bed in this chapter.
17.2 NEED AND LOCATION
Each grade has its own unique characteristics. Highway alignment, gradient, length and descent speed contribute to
the potential for out-of-control vehicles. For existing highways, operational problems on a downgrade will often be
reported by law enforcement officials, truck drivers or the general public. A field review of a specific grade may
reveal damaged guide rail, gouged pavement surfaces or spilled oil indicating locations where drivers of heavy
vehicles had difficulty negotiating a downgrade. For existing facilities, an escape ramp should be provided as soon
as a need is established. Crash experience (or, for new facilities, use crash experience on similar facilities) and truck
operations on the grade combined with engineering judgment are used frequently to determine the need for a truck
escape ramp. Often the impact of potential runaway trucks on adjacent activities or population centers will provide
sufficient reason to construct an escape ramp. Likewise, for Interstate highways on extended lengths of maximum
or near maximum descending grades, emergency escape ramps should be added where an evaluation indicates they
are required.
Unnecessary escape ramps should be avoided. For example, a second escape ramp should not be needed just
beyond the curve that created the need for the initial ramp.
While there are no universal guidelines available for new and existing facilities, a variety of factors should be
considered in selecting the specific site for an escape ramp. Each location presents a different array of design needs;
factors that should be considered include topography, length and percent of grade, potential speed, economics,
environmental impact and crash experience. Ramps should be located to intercept the greatest number of runaway
vehicles, such as at the bottom of the grade and at intermediate points along the grade where an out-of-control
vehicle could cause a catastrophic crash.
A technique for new and existing facilities available for use in analyzing operations on a grade, in addition to crash
analysis, is the Grade Severity Rating System. The system uses a predetermined brake temperature limit (260 °C
(500 °F)) to establish a safe descent speed for the grade. It also can be used to determine expected brake
temperatures at 0.8 km (0.5 mi) intervals along the downgrade. The location where brake temperatures exceed the
limit indicates the point that brake failures can occur, leading to potential runaways.
Escape ramps generally may be built at any practical location where the main road alignment is tangent. They
should be built in advance of horizontal curves that cannot be negotiated safely by an out-of-control vehicle and in
advance of populated areas. Escape ramps should exit to the right of the roadway. On divided multilane highways,
where a left exit may appear to be the only practical location, difficulties may be expected by the refusal of vehicles
in the left lane to yield to out-of-control vehicles attempting to change lanes.
Although crashes involving runaway trucks usually can occur at various sites along a grade, locations having
multiple crashes should be analyzed in detail. Analysis of crash data pertinent to a prospective site should include
evaluation of the section of highway immediately uphill including the amount of curvature traversed and distance to
and radius of the adjacent curve.
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An integral part of the evaluation should be the determination of the maximum speed that an out-of-control vehicle
could attain at the proposed site. This highest obtainable speed can then be used as the minimum design speed for
the ramp. The 130 to 140 km/h (80 to 90 mph) entry speed, recommended for design, is intended to represent an
extreme condition and therefore should not be used as the basis for selecting locations of escape ramps. Although
the variables involved make it impractical to establish a maximum truck speed warrant for location of escape ramps,
it is evident that anticipated speeds should be below the range used for design. The principal factor in determining
the need for an emergency escape ramp should be the safety of the other traffic on the roadway, the driver of the outof-control vehicle and the residents along and at the bottom of the grade. An escape ramp, or ramps if the conditions
indicate the need for more than one, should be located wherever grades are of a steepness and length that present a
substantial risk of runaway trucks and topographic conditions will permit construction.
17.3 TYPES OF EMERGENCY ESCAPE RAMPS
Emergency escape ramps have been classified in a variety of ways. Three broad categories used to classify ramps
are gravity, sandpile and arrester bed. Within these broad categories, four basic emergency escape ramp designs
predominate. These designs are the sandpile and three types of arrester beds, classified by grade of the arrester bed:
descending grade, horizontal grade and ascending grade. Typical escape ramps are shown in the 2004 AASHTO
Green Book, Chapter 3, Exhibits 3-67 and 3-68.
The gravity ramp has a paved or densely compacted aggregate surface, relying primarily on gravitational forces to
slow and stop the runaway. Rolling resistance forces contribute little to assist in stopping the vehicle. Gravity
ramps are usually long and steep and are constrained by topographic controls and costs. While a gravity ramp stops
forward motion, the paved surface cannot prevent the vehicle from rolling back down the ramp grade and
jackknifing without a positive capture mechanism. Therefore, the gravity ramp is the least desirable of the escape
ramp types.
Sandpiles, composed of loose, dry sand dumped at the ramp site, are usually no more than 120 m (400 ft) in length.
The influence of gravity is dependent on the slope of the surface. The increase in rolling resistance is supplied by
loose sand. Deceleration characteristics of sandpiles are usually severe and the sand can be affected by weather.
Because of the deceleration characteristics, the sandpile is less desirable than the arrester bed. However, at locations
where inadequate space exists for another type of ramp, the sandpile may be appropriate because of its compact
dimensions.
Descending-grade arrester-bed escape ramps are constructed parallel and adjacent to the through lanes of the
highway. These ramps use loose aggregate in an arrester bed to increase rolling resistance to slow the vehicle. The
gradient resistance acts in the direction of vehicle movement. As a result, the descending-grade ramps can be rather
lengthy because the gravitational effect is not acting to help reduce the speed of the vehicle. The ramp should have
a clear, obvious return path to the highway so drivers who doubt the effectiveness of the ramp will feel they will be
able to return to the highway at a reduced speed.
Where the topography can accommodate, a horizontal-grade arrester-bed escape ramp is another option.
Constructed on an essentially flat gradient, the horizontal-grade ramp relies on the increased rolling resistance from
the loose aggregate in an arrester bed to slow and stop the out-of-control vehicle, since the effect of gravity is
minimal. This type of ramp is longer than the ascending-grade arrester bed.
The most commonly used escape ramp is the ascending-grade arrester bed. Ramp installations of this type use
gradient resistance to its advantage, supplementing the effects of the aggregate in the arrester bed, and in general,
reducing the length of ramp needed to stop the vehicle. The loose material in the arresting bed increases the rolling
resistance, as in the other types of ramps, while the gradient resistance acts downgrade, opposite to the vehicle
movement. The loose bedding material also serves to hold the vehicle in place on the ramp grade after it has come
to a safe stop.
Each of the ramp types is applicable to a particular situation where an emergency escape ramp is desirable and
should be compatible with established location and topographic controls at possible sites. The procedures used for
analysis of truck escape ramps are essentially the same for each of the categories or types identified. The rolling
resistance factor for the surfacing material used in determining the length needed to slow and stop the runaway
safely is the difference in the procedures.
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Publication 13M (DM-2)
17.4 ELEMENTS OF DESIGN
The principal factor in determining the need for an emergency escape ramp should be the safety of the other traffic
on the roadway, the driver of the out-of-control vehicle and the residents along and at the bottom of the grade.
To safely stop an out-of-control vehicle, the length of an escape ramp should be sufficient to dissipate the kinetic
energy of the moving vehicle. A "last chance" device at the end of the ramp, such as a mound or a row of barrels,
should be considered when the consequences of leaving the end of ramp are serious.
There are numerous elements which affect the performance of emergency escape ramps. The depth, length and
slope as well as the gradation, density and type of material are important factors in the performance of an arresting
bed.
Resistance forces limit the maximum speed of an out-of-control vehicle. Speeds in excess of 130 to 140 km/h (80 to
90 mph) will rarely, if ever, be attained. For the escape ramp to be effective, it must stop the largest vehicle
expected to use the ramp; generally a truck, such as a WB-15 (WB-50) or a WB-18 (WB-60).
Access to the ramp should be made obvious by exit signing, with sufficient sight distance in advance, to allow the
driver of an out-of-control vehicle time to react, so as to minimize the possibility of missing the ramp. Advance
signing is needed to inform drivers of the existence of an escape ramp and to prepare drivers well in advance of the
decision point so that they will have enough time to decide whether or not to use the escape ramp. Regulatory signs
near the entrance should be used to discourage other motorists from entering, stopping, or parking at or on the ramp.
The path of the ramp should be delineated to define ramp edges and provide nighttime direction. Illumination of the
approach and ramp is desirable.
Design recommendations for emergency escape ramps are divided into the following subsections: (1) Basic Bed
Length, (2) Barrels, (3) Mounds, (4) Length Design with Combination of Bed, Mounds and Barrels, (5) Bed Design
and (6) Incidental Items.
A. Basic Bed Length. The basic bed length (L) required without mounds or barrels is given by a third-order
equation:
L = A + BV + CV2 + DV3
(Eq. 17-1)
where:
L
V
A,B,C,D
=
=
=
Basic bed length (m (ft)).
Entry speed (km/h (mph)).
Constants given in Table 17.1.
The above equation is used to calculate the basic bed length for entry speeds up to 140 km/h (90 mph). However,
the values of the constants used with the equation are different. In Table 17.1, a set of values is given for entry
speeds from 50 to 100 km/h (30 to 60 mph) and another set of values for entry speeds from 101 to 140 km/h (61 to
90 mph).
To calculate L, the entry speed and bed grade are chosen, and then the constants can be determined from Table 17.1.
METRIC EXAMPLE: To find the length required for an entry speed of 100 km/h at 0% grade and 10% grade, the
constants are first found from Table 17.1.
0% grade
A0 =
B0 =
C0 =
D0 =
0.992 202 49
0.032 091 92
0.005 095 26
0.000 047 68
10% grade
A10 =
-1.804 566 84
B10 =
0.118 240 57
C10 =
0.003 723 85
D10 =
0.000 031 98
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Publication 13M (DM-2)
By substituting these values into Equation 17-1, lengths of 103 m for a bed with 0% grade and 79 m for a bed with
10% grade are found. As shown here, if conditions dictate a shorter bed length, the grade of the bed is required to
be steeper.
1. Graphs for Designing the Bed Length. Figure 17.1 is a plot of Equation 17-1 using the values from Table
17.1 and thus can be used as an alternative to Equation 17-1 for designing bed length. In the example given
above, the 100 km/h line would be followed to the 0% grade curve where the length of 103 m is indicated. For
a 10% slope, the 100 km/h line would be followed to the 10% grade curve, where the length of 79 m, is shown.
The basic bed length is also given in Table 17.2 which results from using Equation 17-1 and the constants from
Table 17.1. The basic bed length is given for entry speeds of 2 km/h increments from 50 to 140 km/h. This
table is to be used as a quick reference during preliminary design and for review of final plans for emergency
escape ramps.
ENGLISH EXAMPLE: To find the length required for an entry speed of 60 mph at 0% grade and 10% grade, the
constants are first found from Table 17.1.
0% grade
A0 =
B0 =
C0 =
D0 =
3.2552575
0.16944534
0.04329616
0.00065202
10% grade
A10 =
-5.92049487
B10 =
0.62431019
C10 =
0.03164279
D10 =
0.00043730
By substituting these values into Equation 17-1, lengths of 310 ft for a bed with 0% grade and 240 ft for a bed with
10% grade are found. As shown here, if conditions dictate a shorter bed length, the grade of the bed is required to
be steeper.
1. Graphs for Designing the Bed Length. Figure 17.1 is a plot of Equation 17-1 using the values from Table
17.1 and thus can be used as an alternative to Equation 17-1 for designing bed length. In the example given
above, the 60 mph line would be followed to the 0% grade curve where the length of 310 ft is indicated. For a
10% slope, the 60 mph line would be followed to the 10% grade curve, where the length of 260 ft, is shown.
The basic bed length is also given in Table 17.2 which results from using Equation 17-1 and the constants from
Table 17.1. The basic bed length is given for entry speeds of 2 mph increments from 30 to 90 mph. This table
is to be used as a quick reference during preliminary design and for review of final plans for emergency escape
ramps.
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Publication 13M (DM-2)
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Chapter 17 - Emergency Escape Ramps
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Chapter 17 - Emergency Escape Ramps
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TABLE 17.1 (METRIC)
VALUES FOR CONSTANTS USED TO CALCULATE
BASIC BED LENGTH IN EQUATION 17-1
50 TO 100 km/h
CONSTANT
PERCENT GRADE OF BED
-5
0
5
10
15
20
25
A
-15.172 944 00
0.992 202 49
-4.921 267 79
-1.804 566 84
0.492 795 03
1.457 464 15
7.102 965 24
B
0.782 196 97
0.032 091 92
0.264 322 13
0.118 240 57
-0.010 606 62
-0.039 982 48
-0.303 254 64
C
-0.006 001 88
0.005 095 26
0.001 874 60
0.003 723 85
0.005 658 15
0.005 612 71
0.009 278 71
D
0.000 120 66
0.000 047 68
0.000 049 02
0.000 031 98
0.000 015 32
0.000 012 06
-0.000 008 81
101 TO 140 km/h
CONSTANT
PERCENT GRADE OF BED
5
10
-5
0
15
20
25
A
-282.525 459 84
-86.921 166 05
-22.415 263 66
-23.581 269 96
-30.226 931 56
-4.254 939 42
-3.029 281 62
B
8.624 979 66
2.744 483 34
0.794 359 49
0.736 911 89
0.836 898 03
0.177 542 32
0.093 922 63
C
-0.083 083 36
-0.023 107 90
-0.003 412 29
-0.002 057 50
-0.002 419 34
0.002 973 24
0.003 926 11
D
0.000 374 69
0.000 146 79
0.000 066 36
0.000 049 74
0.000 042 33
0.000 022 58
0.000 015 36
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Chapter 17 - Emergency Escape Ramps
Publication 13M (DM-2)
TABLE 17.1 (ENGLISH)
VALUES FOR CONSTANTS USED TO CALCULATE
BASIC BED LENGTH IN EQUATION 17-1
30 TO 60 mph
CONSTANT
PERCENT GRADE OF BED
5
10
15
-5
0
20
25
A
-49.78
3.2552575
-16.14589169
-5.92049487
1.61678158
4.78170653
23.30369173
B
4.13
0.16944534
1.39562086
0.62431019
-0.05600296
-0.21110752
-1.60118449
C
-0.051
0.04329616
0.01592913
0.03164279
0.04807917
0.04769312
0.07884436
D
0.00165
0.00065202
0.00067035
0.00043730
0.00020949
0.00016497
-0.00012045
61 TO 90 mph
CONSTANT
PERCENT GRADE OF BED
5
10
15
-5
0
20
25
A
-926.9208
-285.1744293
-73.54089126
-77.36637125
-99.16972295
-13.9597750
-9.93858798
B
45.53989258
14.49087202
4.19421811
3.8908948
4.41882161
0.93742345
0.49591149
C
-0.70598725
-0.19635557
-0.02899535
-0.01748326
-0.02055797
0.02526466
0.03336147
D
0.00512399
0.00200734
0.00090754
0.00068014
0.00057884
0.00030885
0.00021004
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Chapter 17 - Emergency Escape Ramps
Publication 13M (DM-2)
TABLE 17.2 (METRIC)
BASIC BED LENGTH FOR ENTRY
SPEED FOR SOME BED GRADES
BASIC BED LENGTH (m)
ENTRY SPEED
(km/h)
50
52
54
56
58
60
62
64
66
68
70
72
74
76
78
80
82
84
86
88
90
92
94
96
98
100
102
104
106
108
110
112
114
116
118
120
PERCENT GRADE OF BED
-5
0
5
10
15
20
25
24
26
29
31
34
36
39
42
45
48
52
55
59
63
67
71
75
80
84
89
95
100
106
111
117
124
130
137
144
152
160
168
176
185
194
204
21
23
25
27
29
32
34
36
39
42
45
48
51
54
57
61
64
68
72
76
80
84
89
93
98
103
108
114
119
125
131
137
143
150
156
163
19
21
23
24
26
28
30
33
35
37
40
42
45
48
50
53
56
60
63
66
70
73
77
81
85
89
94
98
102
107
112
117
122
127
133
138
17
19
20
22
24
26
27
29
31
34
36
38
40
43
45
48
51
53
56
59
62
65
69
72
76
79
83
87
91
95
99
103
107
112
116
121
16
17
19
20
22
24
25
27
29
31
33
35
37
39
41
44
46
49
51
54
57
59
62
65
68
71
75
78
82
85
89
93
96
100
104
109
15
16
18
19
20
22
23
25
27
28
30
32
34
36
38
40
43
45
47
50
52
55
57
60
63
66
69
72
75
78
81
85
88
92
95
99
14
15
16
18
19
20
22
23
25
27
28
30
32
34
36
38
40
42
44
46
49
51
53
56
58
61
64
66
69
72
75
78
81
85
88
91
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TABLE 17.2 (METRIC)(CONTINUED)
BASIC BED LENGTH FOR ENTRY
SPEED FOR SOME BED GRADES
BASIC BED LENGTH (m)
ENTRY SPEED
(km/h)
122
124
126
128
130
132
134
136
138
140
142
144
146
148
150
152
154
156
158
160
162
164
166
168
170
172
174
176
178
180
182
184
186
188
190
PERCENT GRADE OF BED
-5
0
5
10
15
20
25
213
224
235
246
258
270
283
296
310
325
340
355
372
389
406
425
444
464
484
505
527
550
574
598
623
650
677
705
733
763
794
826
858
892
927
171
178
186
194
202
210
219
228
238
247
257
267
278
289
300
312
324
336
349
362
375
389
403
418
433
448
464
481
497
514
532
550
569
588
607
144
150
156
163
169
176
182
189
197
204
212
219
227
236
244
253
261
270
280
289
299
309
319
329
340
351
362
373
385
397
409
422
434
447
461
126
131
136
141
147
152
158
164
170
176
182
188
195
202
209
216
223
230
238
245
253
261
270
278
287
295
304
314
323
332
342
352
362
373
383
113
117
121
126
131
135
140
145
150
156
161
167
172
178
184
190
196
202
209
215
222
229
236
243
250
258
265
273
281
289
297
306
314
323
332
103
107
110
115
119
123
127
132
136
141
146
150
155
160
165
171
176
182
187
193
199
204
210
217
223
229
236
242
249
256
263
270
277
284
292
95
98
102
106
109
113
117
121
125
129
133
138
142
147
151
156
161
165
170
175
181
186
191
196
202
207
213
219
225
231
237
243
249
255
262
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TABLE 17.2 (ENGLISH)
BASIC BED LENGTH FOR ENTRY
SPEED FOR SOME BED GRADES
BASIC BED LENGTH (ft)
PERCENT GRADE OF BED
ENTRY SPEED
(mph)
-5
0
5
10
15
20
25
30
32
34
36
38
40
42
44
46
48
50
52
54
56
58
60
62
64
66
68
70
72
74
76
78
80
82
84
86
88
90
92
94
96
98
100
73
84
96
110
124
139
156
174
193
213
235
259
284
311
340
371
404
439
476
516
559
605
654
706
762
821
885
954
1027
1105
1189
1279
1375
1477
1586
1702
65
74
85
96
108
121
135
150
166
183
202
221
241
263
286
311
337
364
393
423
456
490
525
563
603
645
690
736
786
837
892
949
1009
1073
1139
1209
58
67
76
86
97
108
120
133
147
162
177
194
211
230
249
270
291
314
338
363
389
417
446
476
508
541
576
612
650
689
731
774
819
866
915
966
53
61
69
78
87
98
109
120
132
145
159
174
189
205
222
240
259
278
299
320
343
366
390
416
443
470
499
529
561
593
627
662
699
737
776
817
49
56
63
72
80
90
100
110
121
133
145
158
172
186
201
217
234
251
269
288
308
329
350
372
395
419
444
470
497
525
554
584
615
647
681
715
46
52
59
67
75
83
92
102
112
123
134
146
158
172
185
200
215
230
247
264
281
300
319
339
359
381
403
426
450
475
500
526
554
582
611
641
43
49
55
62
70
78
86
95
105
115
125
136
148
160
172
185
199
214
229
244
260
277
294
313
331
351
371
392
413
435
458
482
506
532
558
584
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TABLE 17.2 (ENGLISH)(CONTINUED)
BASIC BED LENGTH FOR ENTRY
SPEED FOR SOME BED GRADES
BASIC BED LENGTH (ft)
PERCENT GRADE OF BED
ENTRY SPEED
(mph)
-5
0
5
10
15
20
25
102
104
106
108
110
112
114
116
118
120
1824
1953
2088
2228
2372
2521
2671
2824
2976
3128
1282
1358
1438
1521
1608
1699
1792
1890
1990
2093
1019
1074
1131
1190
1252
1316
1382
1451
1521
1595
859
903
948
995
1043
1093
1145
1198
1253
1309
750
787
825
864
904
945
988
1032
1077
1124
672
703
736
770
805
840
877
915
954
994
612
640
669
699
730
762
794
828
862
897
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B. Barrels. Impact attenuator barrels can be used to help decelerate trucks where insufficient space is available
for proper bed length. The following three equations give the change in speed, average deceleration load and time to
travel the 0.9 m (3 ft) through a row of barrels:
Vf
=
DVe
g
=
0.004 298 8 (D2 - 1)Ve2
t
=
6.584 / (D + l)Ve
g
=
0.011134 (D2 - 1)Ve2
t
=
4.091 / (D + l)Ve
(Eq. 17-2)
METRIC:
ENGLISH:
where:
Vf
Ve
D
g
t
=
=
=
=
=
Exit speed after barrel row (km/h (mph)).
Entry speed into barrel row (km/h (mph)).
Factor given in Table 17.3.
Deceleration, g.
Time to travel length of barrels (s).
METRIC EXAMPLE:
Three or more barrels across by four rows deep, would permit an additional reduction of 50 km/h when combined
with the last 4.4 m of the bed for a 36 300 kg vehicle and would cause a 4.7-g load. Otherwise, 21 m of bed without
the barrel barrier would be required to achieve the same results at 0% grade. Because of the larger deceleration
loads at speeds above 70 km/h, a design as given in Figure 17.2 is suggested to reduce the maximum to below 9-g.
This design is good for vehicles traveling as fast as 100 km/h. If speeds are higher, the designer should follow the
procedures given in the next section, on barrel curves, to design the appropriate configuration.
ENGLISH EXAMPLE:
Three or more barrels across by four rows deep, would permit an additional reduction of 30 mph when combined
with the last 14 ft of the bed for a 80,000 lb vehicle and would cause a 4.7-g load. Otherwise, 65 ft of bed without
the barrel barrier would be required to achieve the same results at 0% grade. Because of the larger deceleration
loads at speeds above 45 mph, a design as given in Figure 17.2 is suggested to reduce the maximum to below 9-g.
This design is good for vehicles traveling as fast as 60 mph. If speeds are higher, the designer should follow the
procedures given in the next section, on barrel curves, to design the appropriate configuration.
1. Graphs for Designing the Barrel Array. The equations given in the previous section are presented
graphically in Figure 17.3. This figure presents, in the same manner as that used in the Energite System
manual for the inertial barrier system, the design for a 36 300 kg (80,000 lb) vehicle using 0.64 m3 (22.6 ft3)
barrels filled with river gravel (AASHTO No. 57). Sand should not be used in the barrels since sand would
contaminate the gravel bed. The maximum deceleration forces acceptable for a vehicle's occupants are 12-g.
The figure gives deceleration force levels up to 12-g. However, a deceleration force of 9-g is desirable for
design. To use Figure 17.3, the same procedure is followed as was used to determine the bed length.
A vehicle impacting a three by three barrel array at 50 km/h (30 mph) can be considered as an example. Using
Figure 17.4, the initial impact speed (50 km/h (30 mph)) can be located on the baseline (point 1). A straight
vertical line can be drawn from that point until it intersects the horizontal line for three-barrel rows. The
deceleration (4.7-g) is read here.
From that point, another line can be drawn parallel to the nearest exit speed line down to the baseline (point 2),
which gives an exit speed of 38 km/h (23 mph) from the first row. The procedure is repeated for the second
row, giving a deceleration of 2.6-g and an exit speed of 28 km/h (17 mph) at point 3. Repeating this procedure
for the third row gives 1.5-g and 21 km/h (12 mph) at exit.
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TABLE 17.3
D FACTORS FOR BARRELS
ROW
ONE BARREL
TWO BARRELS
THREE BARRELS
820 kg
(1800 lb)
0.16
0.09
0.06
2040 kg
(4500 lb)
0.33
0.20
0.14
VEHICLE MASS
(Vehicle Weight)
6580 kg
14 970 kg
(14,500 lb)
(33,000 lb)
0.60
0.78
0.42
0.65
0.35
0.55
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18 600 kg
(41,000 lb)
0.82
0.69
0.60
36 300 kg
(80,000 lb)
0.90
0.81
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For this case, the speed is decreased from 50 km/h to 21 km/h (30 to 12 mph) over 3.0 m (10 ft). Also, from Figure
17.1, 3.0 m (10 ft) of a 0% grade bed will remove 21 km/h (12 mph) from the vehicle speed; i.e., the combination of
three rows of barrels and 3.0 m (10 ft) of bed will remove a total of 50 km/h (30 mph).
In another example (not marked on the figure) of a single row of two barrels with an impact speed of 60 km/h
(40 mph), a 5.3-g deceleration and an exit speed of 49 km/h (33 mph) will be found.
C. Mounds. Mounds, which are depicted by Figure 17.5, are treated in a manner similar to that used for the
barrels, and the same speed change equation is used; however, a different deceleration equation is needed because,
while the average deceleration is calculated in the same manner, the peak is different. Using Equation 17-2, the
equations for mounds were developed and are given below:
Vf = DVe
METRIC:
1.
Full Mound:
gave = 0.001 29 (D2 - 1)Ve2
gpeak = 0.002 34 (D2 - 1)Ve2
t
2.
= 21.945 / (D + 1)Ve
Half Mound:
gave = 0.002 58 (D2 - 1)Ve2
gpeak = 0.004 29 (D2 - 1)Ve2
t
= 10.973 / (D + 1)Ve
ENGLISH:
1.
Full Mound:
gave = 0.00334 (D2 - 1)Ve2
gpeak = 0.00607 (D2 - 1)Ve2
t
2.
= 13.636 / (D + 1)Ve
Half Mound:
gave = 0.00668 (D2 - 1)Ve2
gpeak = 0.01112 (D2 - 1)Ve2
t
= 6.818 / (D + 1) Ve
where:
Vf
Ve
D
g
t
=
=
=
=
=
Exit speed from mound (km/h (mph))
Entry speed into mound (km/h (mph))
Factor given in Table 17.4.
Deceleration, g.
Time to travel mound(s) (s).
17 - 16
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TABLE 17.4
D FACTORS FOR MOUNDS
MOUND
TYPE
FULL
HALF
820 kg
(1800 lb)
0.13
0.23
2040 kg
(4500 lb)
0.28
0.43
VEHICLE MASS
(VEHICLE WEIGHT)
6580 kg
14 970 kg
(14,500 lb)
(33,000 lb)
0.55
0.74
0.71
0.85
18 600 kg
(41,000 lb)
0.78
0.87
36 300 kg
(80,000 lb)
0.87
0.93
Tests have shown that mounds should not be placed nearer than 30 m (100 ft) into the bed. When a truck is
still riding on top of the bed, it will ride up over the mound, giving high vertical acceleration. In the case of the
full-sized mound, the truck can become airborne; however, once the truck sinks into the bed, it generally plows
through the mounds.
Mound usage should be avoided, if possible. If they are used, however, they should be placed in the bed such
that they will be hit at slow speeds, 40 km/h (25 mph) and less. Although barrels are more expensive than
mounds, barrels are recommended.
3. Graphs for Designing Mounds. In the same manner used for the barrel designs, given previously, Figure
17.6 is used in the design of mounds. Figure 17.5 shows the cross sections for a full and a half mound. An
example is shown in Figure 17.7 for a 36 300 kg (80,000 lb) vehicle traveling 70 km/h (45 mph) into a full
mound. It has a peak deceleration of 2.8-g and exits at 61 km/h (38 mph). Although a deceleration force of
2.8-g is acceptable for the driver, the larger forces are on only the first (front) axle, and for a 36 300 kg
(80,000 lb) vehicle, this deceleration gives a force of approximately 1020 kN (240,000 lb).
D. Design with Combination Bed, Mounds and Barrels. A bed design in combination with mounds or barrels
or both requires the use of Figures 17.3, 17.6 and 17.8. Figure 17.8 is a replot of Figure 17.1 with the x and y axes
interchanged. This figure can be used to illustrate the design; an example is shown for the 0% grade.
METRIC EXAMPLE: Suppose a bed is proposed to stop a 36 300 kg vehicle traveling at 100 km/h on a 0% grade.
Consider a mound placed at 30 m and three rows of barrels at the end. For this example, Figures 17.9, 17.10 and
17.11 are used. First, from Figure 17.10, if the speed is reduced to 50 km/h when the barrels are reached, the exit
speed due to the barrels alone is 21 km/h. Since the barrels are 2.7 m long, the 0% curve of Figure 17.11 shows that
the remaining 21 km/h will be eliminated by the bed while traveling the 2.7 m. Thus, enough length of the bed plus
the mound will be needed to get the speed down to 50 km/h.
Using Figure 17.11, a horizontal line should be drawn from the 100 km/h point until the 0% grade line has been
intersected. The 0% grade line is then followed for 30 m from 103 m to 73 m, at which point the full mound has
been reached. A horizontal line is then drawn to find that the speed is 86 km/h. Using Figure 17.9, an entrance
speed of 86 km/h gives an exit speed of 75 km/h. When Figure 17.11 is re-entered at 75 km/h, the 0% curve is
followed to 50 km/h, which is the speed reduction previously found for the three rows of barrels. Thus, the first
30 m of the bed (see Figure 17.11) plus 32 m (from 53 m down to 21 m) plus the 2 m at the end adds up to require a
bed of 64 m.
ENGLISH EXAMPLE: Suppose a bed is proposed to stop a 80,000 lb vehicle traveling at 60 mph on a 0% grade.
Consider a mound placed at 100 ft and three rows of barrels at the end. For this example, Figures 17.9, 17.10, and
17.11 are used. First, from Figure 17.10, if the speed is reduced to 30 mph when the barrels are reached, the exit
speed due to the barrels alone is 12 mph. Since the barrels are 9 ft long, the 0% curve of Figure 17.11 shows that the
remaining 12 mph will be eliminated by the bed while traveling the 9 ft. Thus, enough length of the bed plus the
mound will be needed to get the speed down to 30 mph.
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Using Figure 17.11, a horizontal line should be drawn from the 60 mph point until the 0% grade line has been
intersected. The 0% grade line is then followed for 100 ft from 311 ft to 211 ft, at which point the full mound has
been reached. A horizontal line is then drawn to find that the speed is 51 mph. Using Figure 17.9, an entrance
speed of 51 mph gives an exit speed of 45 mph. When Figure 17.11 is re-entered at 45 mph, the 0% curve is
followed to 30 mph, which is the speed reduction previously found for the three rows of barrels. Thus, the first
100 ft of the bed (see Figure 17.11) plus 105 ft (from 158 ft down to 65 ft) plus the 5 ft at the end adds up to require
a bed of 198 ft.
When space permits, the bed length should be designed as required for the entry speed without regard to reduction in
distance due to barrels or mounds. The barrels in this case would be provided as an added safety benefit at the end
of the ramp.
E. Bed Design. The method for designing the length of an arrester bed was addressed in the previous section,
along with the need to consider the grade of the bed and the use of barrels and of gravel mounds in the overall bed
design. Other characteristics of the bed which must be considered for an effective design are the depth and width of
the bed and drainage features. Also essential to the overall arrester bed operation is the installation of a concrete
anchor block 15 to 30 m (50 to 100 ft) in front of the bed to act as a dead man for the tow vehicle used for truck
extraction.
1. Bed Depth. A minimum of 1070 mm (42 in) is the recommended depth for beds of river gravel. Testing
has shown that a bed with 915 mm (36 in) of river gravel gives the same results as a bed as deep as 2440 mm
(8 ft). The minimum recommended depth includes 150 mm (6 in) to allow for compaction when the gravel
contains many fines, especially if the bed is located where the potential for heavy use is great. Frequent use
results in the significant increase in fines content, which decreases the effectiveness of the bed.
Smooth, rounded, uncrushed gravel of approximately a single size is the most effective arrester bed material.
The best size is approximately 13 mm (0.5 in) in diameter.
The river gravel graded to AASHTO No. 57 was found to be the best of those materials tested if it had been
washed so that fines were removed. A greater percentage of larger or smaller stones decreased the
effectiveness and increased planning. Crushed stone should not be used because it provides a drag factor of
about half that of rounded gravel and compacts more quickly. To be effective, crushed aggregate would
require longer beds and fluffing almost weekly.
Rounded river gravel produces higher decelerations than the more angular crushed aggregate because the truck
sinks into the river gravel more, transferring more energy to the stones over a shorter distance.
2. Bed Width. In general, a width of 6.6 to 7.5 m (22 to 25 ft) is recommended on the basis of an entry
design which has a horizontal straightaway of at least one truck length, directing the vehicle in a straight line
through the center of the bed. If the top of the bed is at ground level, a minimum width of 6.6 m (22 ft) would
be sufficient. Conversely, if either side of the bed is a drop-off of any height, the bed should be wider. The
vehicle's direction of momentum as it enters the bed is the direction it will travel through the bed; this direction
should be the bed center line.
3. Bed Drainage. Proper drainage so that water does not stand in the bed is important. A 305 mm (12 in)
base layer of large (at least 75 mm (3 in) in diameter) crushed limestone aggregate (AASHTO No. 1) will
effectively drain the arrester bed. The stones should be confined to the layer by covering them with geotextile
material (Type 3) to separate the larger stones from the river gravel. The cross slope of the base should be
toward one side, with either subgrade drains or a crown for removing any water from the arrester layer. The
river gravel covering this sloping base layer should have no cross slope at the top surface; i.e., the bed should
be filled such that the top surface has no cross slope.
F.
Incidental Items.
1. Service Road. A service road located adjacent to the arrester bed is recommended so the tow and
maintenance vehicles can use it without becoming trapped in the bedding material. The typical width of this
road is 3.0 m (10 ft).
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2. Concrete Anchor Block. Every escape ramp must include a concrete anchor block to which a tow vehicle
can attach when it pulls an arrested vehicle from the gravel bed. The anchor provides a necessary dead man
which can withstand the retraction loads required to pull an arrested vehicle from the ramp bed. Retraction
loads range from 25-100% of the gross vehicle mass (weight) and are generally about 50%. Thus, for example,
a load of more than 18 140 kg (40,000 lb) may be required to extract a loaded tractor trailer. During some
tests, towing service personnel who did not believe the anchor would be necessary found that the tow truck,
rather than the captured vehicle, was moved during extraction attempts. In one such case, the paved approach
was damaged. Thus, it is recommended that every escape ramp include an anchor block such as that shown in
Figure 17.12, and should be located at the center and flush with the road surface 15 to 30 m (50 to 100 ft) from
the start of the bed. Additional blocks may be needed, depending on the length of the escape ramp. Towing
services should be required to use the anchor block.
3. Gravel Fluffer. To preserve its original density, the gravel bed should be fluffed by a device capable of
breaking up the compacted areas. A typical mechanical device is shown on Figure 17.13. This device,
designed by PTI, consists of a sled with prongs that extend down into the gravel bed. As the sled is pulled
through the bed, its prongs break up the compacted areas. One important element of a fluffer is adequate mass
(weight) to break up the compacted area at the required depth.
4. Extraction. Extraction should be performed with a wrecker and a winch. The front of the wrecker must
be chained to a dead man anchor block, and the winch must be used with a block and tackle that has at least a
two to one mechanical advantage.
After the vehicle being extracted from the bed begins to move, it can be raised onto 50 mm × 150 mm
(2 in × 6 in) boards to greatly reduce the drawbar pull required to remove the vehicle from the bed and,
correspondingly, the pull loads on the vehicle and its axles. At least two boards per wheel set should be used
so that, as a wheel rolls off one board, it will roll onto the next board. As the wheel then rolls clear of the first
board, that board can be placed in front of the second board, which can then be moved in front of the first
board, and so on, in a leapfrog fashion.
When the captured vehicle has wheel flaps, an important note of caution must be heeded. Flaps must be
removed or tied such that they will not wrap around the wheel between the stones and the tire. Only a turning
wheel will ride up onto the aggregate rather than dig into them, and a barrier between the stones and tire will
prevent the wheel from turning. Consequently, when the wheel flaps provide such a barrier, the wheel
generally digs deeper into the stones, thus creating a need for greater force to pull the vehicle.
G. Maintenance. All of the beds were found to compact with time depending on the durability of the stone.
During the design phase, consideration should be given to access by maintenance vehicles to fluff and maintain
escape ramps in good operating condition. For maintenance details and requirements, refer to the Maintenance
Manual.
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Chapter 17 - Emergency Escape Ramps
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1219
279
267
559
SECTION B-B
2 Lids - 610 x 914 x 19
Steel Plates Opening
51 Diameter
254
203
102
51
51
19 83
38
A
B
76
203
B
610
203
51 51
279
A
610
51 51
279
203
38 PVC Pipe
Outlet Thru
Embankment
254
2 No. 43 Bars at 3048
305 Bend on Bars
305
SECTION A-A
1829
Note: All Dimensions are in Millimeters (mm) Except as Noted.
FIGURE 17.12 (METRIC)
Design of a Concrete Anchor Block Dead Man
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4'
11"
SECTION B-B
2 Lids - 2' x 3' x 0.75"
Steel Plates Opening
2" Diameter
10.5"
1'-10"
10"
4"
8"
2"
0.75"
3.25"
1.50"
2"
A
B
3"
8"
B
2'
8"
2" 2"
11"
A
2'
2" 2"
11"
8"
1.5" PVC Pipe
Outlet Thru
Embankment
10"
2 No. 12 Bars at 10'
1' Bend on Bars
1'
SECTION A-A
6'
FIGURE 17.12 (ENGLISH)
Design of a Concrete Anchor Block Dead Man
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Chapter 17 - Emergency Escape Ramps
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Chapter 17 - Emergency Escape Ramps
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