The purpose of an impact threshold curve is to provide one tool to help determine whether the “secondary” or unintentional impacts of Neighborhood Traffic Management Program (NTMP) projects are acceptable. These impacts are frequently in the form of increased traffic on adjacent, non-project, Local Service streets. Impact limitations are expressed as a curve because the level of impact that is considered acceptable will vary, depending on the characteristics of the street being impacted by the NTMP project.
Among the benefits of employing an impact threshold curve are:
a) residents of adjacent non-project streets are provided with some assurance that traffic problems on one Local Service street will not be solved by simply shifting the problem to other Local Service streets;b) the curve can be translated into a table where the impact limit on any given street can be quickly and easily identified; andc) the impact curve provides a quantifiable and objective standard for measuring the success or failure of a project.
The Citizen Advisory Committee recommended that the following guidelines be followed in establishing numeric impact limitations on non-project local service streets.
1. The standard impact curve for NTMP projects should be expressed in terms of total traffic volume, i.e., vehicles per day (vpd). The parameters of the curve should be:
a) It should have a floor of at least 150 vehicles per day. In other words, an increase of up to 150 vehicles per day as a result of an NTMP project is acceptable on any street(1), regardless of its prior volume.b) The curve should have a ceiling of no more than 400 vpd, i.e., no increase of more than 400 vpd on any Local Service street.
Using a spreadsheet and a set of formulas, these parameters have be used to established a standard NTMP Impact Threshold Curve. This curve is shown below.
2. Because of the margin of error inherent in the collection of traffic volume data due to machine error and daily volume fluctuation, the curve should be presented as a wide band or range rather than a specific line. This allows the error margin to be accommodated within the range.
Therefore, two supplementary curves, one on either side of the standard curve, should be shown along with the standard curve. These “margin of error” curves should be plus or minus 50 vehicles per day or 10 percent of the measured pre-test volume, whichever is greater.
The “standard” curve then becomes the “median” curve within a range. An increase in traffic volume that falls between the median curve and the lower curve would probably be acceptable. An increase that falls between the median and the upper curve would possibly be acceptable. An increase that falls above the upper curve would clearly not be acceptable.
3. The standard impact curve may be modified for application to a particular NTMP project based on consideration of the following:
a) The ratio of local to non-local traffic on the project street and adjacent streets.b) The percentage of the rerouted traffic that is local vs. non-local.c) The pre-existing traffic volume on the project street.d) The proximity of arterial routes that can absorb rerouted traffic.e) Peak hour volumes.f) Truck traffic.
Modifications to the “standard” or “median” impact curve would automatically result in adjustments to the upper and lower “margin of error” curves.
An impact threshold curve is only one tool for judging whether a project’s impact on adjacent streets is acceptable. In a sense, the curve describes impact goals for the project. An increase in traffic volume that exceeds the impact threshold described by the curve is not necessarily fatal for a project, unless the project traffic committee has chosen to make that commitment. Other, more qualitative criteria should also be used to help determine whether a project’s secondary or unintended impacts are acceptable.
NTMP Impact Threshold Curve
Describes Acceptable Traffic Volume Increases on Non-Project Streets
CAC/TEC Recommendations April 24, 1991
Set Parameters
CEILING 400 (Peak Increase) at: 2,000 Pre-Test Volume
FLOOR 150 (Floor Increase) at 0 Pre-Test Volume
COLLAPSE 1,123 Exponent Factor for Tail End of Base Curve (Set this parameter at bottom of spread sheet.)
Pre-Test
Volume
|
Base Sine Curve | Acceptable Net Increase | Net Increase in Percent | ||||
Low | Base | High | Low | Base | High | ||
50 | 150 | 100 | 150 | 200 | 200% | 300% | 400% |
100 | 152 | 102 | 152 | 202 | 102% | 152% | 202% |
125 | 152 | 102 | 152 | 202 | 82% | 122% | 162% |
150 | 153 | 103 | 153 | 203 | 69% | 102% | 136% |
175 | 155 | 105 | 155 | 205 | 60% | 88% | 117% |
200 | 156 | 106 | 156 | 206 | 53% | 78% | 103% |
225 | 158 | 108 | 158 | 208 | 48% | 70% | 92% |
250 | 160 | 110 | 160 | 210 | 44% | 64% | 84% |
275 | 161 | 111 | 161 | 211 | 41% | 59% | 77% |
300 | 164 | 114 | 164 | 214 | 38% | 55% | 71% |
325 | 166 | 116 | 166 | 216 | 36% | 51% | 66% |
350 | 168 | 118 | 168 | 218 | 34% | 48% | 62% |
375 | 171 | 121 | 171 | 221 | 32% | 46% | 59% |
400 | 174 | 124 | 174 | 224 | 31% | 43% | 56% |
425 | 177 | 127 | 177 | 227 | 30% | 42% | 53% |
450 | 180 | 130 | 180 | 230 | 29% | 40% | 51% |
475 | 183 | 133 | 183 | 233 | 28% | 39% | 49% |
500 | 187 | 137 | 187 | 237 | 27% | 37% | 47% |
525 | 190 | 138 | 190 | 243 | 26% | 36% | 46% |
550 | 194 | 139 | 194 | 249 | 25% | 35% | 45% |
575 | 198 | 140 | 198 | 255 | 24% | 34% | 44% |
600 | 202 | 142 | 202 | 262 | 24% | 34% | 44% |
625 | 206 | 143 | 206 | 268 | 23% | 33% | 43% |
650 | 210 | 145 | 210 | 275 | 22% | 32% | 42% |
675 | 214 | 146 | 214 | 281 | 22% | 32% | 42% |
700 | 218 | 148 | 218 | 288 | 21% | 31% | 41% |
725 | 223 | 150 | 223 | 295 | 21% | 31% | 41% |
750 | 227 | 152 | 227 | 302 | 20% | 30% | 40% |
775 | 232 | 154 | 232 | 309 | 20% | 30% | 40% |
800 | 236 | 156 | 236 | 316 | 20% | 30% | 40% |
825 | 241 | 159 | 241 | 324 | 19% | 29% | 39% |
850 | 246 | 161 | 246 | 331 | 19% | 29% | 39% |
875 | 251 | 163 | 251 | 338 | 19% | 29% | 39% |
900 | 255 | 165 | 255 | 345 | 18% | 28% | 38% |
925 | 260 | 168 | 260 | 353 | 18% | 28% | 38% |
950 | 265 | 170 | 265 | 360 | 18% | 28% | 38% |
975 | 270 | 173 | 270 | 368 | 18% | 28% | 38% |
1000 | 275 | 175 | 275 | 375 | 18% | 28% | 38% |
1025 | 280 | 177 | 280 | 382 | 17% | 27% | 37% |
1050 | 285 | 180 | 285 | 390 | 17% | 27% | 37% |
1075 | 290 | 182 | 290 | 397 | 17% | 27% | 37% |
1100 | 295 | 185 | 295 | 405 | 17% | 27% | 37% |
1125 | 299 | 187 | 299 | 412 | 17% | 27% | 37% |
1150 | 304 | 189 | 304 | 419 | 16% | 26% | 36% |
1175 | 309 | 191 | 309 | 426 | 16% | 26% | 36% |
1200 | 314 | 194 | 314 | 434 | 16% | 26% | 36% |
1225 | 318 | 196 | 318 | 441 | 16% | 26% | 36% |
1250 | 323 | 198 | 323 | 448 | 16% | 26% | 36% |
1275 | 327 | 200 | 327 | 455 | 16% | 26% | 36% |
1300 | 332 | 202 | 332 | 462 | 16% | 26% | 36% |
1325 | 336 | 204 | 336 | 469 | 15% | 25% | 35% |
1350 | 340 | 205 | 340 | 475 | 15% | 25% | 35% |
1375 | 344 | 207 | 344 | 482 | 15% | 25% | 35% |
1400 | 348 | 208 | 348 | 488 | 15% | 25% | 35% |
1425 | 352 | 210 | 352 | 495 | 15% | 25% | 35% |
1450 | 356 | 211 | 356 | 501 | 15% | 25% | 35% |
1475 | 360 | 212 | 360 | 507 | 14% | 24% | 34% |
1500 | 363 | 213 | 363 | 513 | 14% | 24% | 34% |
1525 | 367 | 214 | 367 | 519 | 14% | 24% | 34% |
1550 | 370 | 215 | 370 | 525 | 14% | 24% | 34% |
1575 | 373 | 216 | 373 | 531 | 14% | 24% | 34% |
1600 | 376 | 216 | 376 | 536 | 14% | 24% | 34% |
1625 | 379 | 216 | 379 | 541 | 13% | 23% | 33% |
1650 | 382 | 217 | 382 | 547 | 13% | 23% | 33% |
1675 | 384 | 217 | 384 | 552 | 13% | 23% | 33% |
1700 | 386 | 216 | 386 | 556 | 13% | 23% | 33% |
1725 | 389 | 216 | 389 | 561 | 13% | 23% | 33% |
1750 | 390 | 215 | 390 | 565 | 12% | 22% | 32% |
1775 | 392 | 215 | 392 | 570 | 12% | 22% | 32% |
1800 | 394 | 214 | 394 | 574 | 12% | 22% | 32% |
1825 | 395 | 213 | 395 | 578 | 12% | 22% | 32% |
1850 | 397 | 212 | 397 | 582 | 11% | 21% | 31% |
1875 | 398 | 210 | 398 | 585 | 11% | 21% | 31% |
1900 | 398 | 208 | 398 | 588 | 11% | 21% | 31% |
1925 | 399 | 207 | 399 | 592 | 11% | 21% | 31% |
1950 | 400 | 205 | 400 | 595 | 10% | 20% | 30% |
1975 | 400 | 202 | 400 | 597 | 10% | 20% | 30% |
2000 | 400 | 200 | 400 | 600 | 10% | 20% | 30% |
2025 | 400 | 197 | 400 | 602 | 10% | 20% | 30% |
2050 | 400 | 195 | 400 | 605 | 9% | 19% | 29% |
2075 | 399 | 192 | 399 | 607 | 9% | 19% | 29% |
2100 | 398 | 188 | 398 | 608 | 9% | 19% | 29% |
2125 | 398 | 185 | 398 | 610 | 9% | 19% | 29% |
2150 | 397 | 182 | 397 | 612 | 8% | 18% | 28% |
2175 | 395 | 178 | 396 | 613 | 8% | 18% | 28% |
2200 | 394 | 175 | 395 | 615 | 8% | 18% | 28% |
2225 | 392 | 171 | 394 | 616 | 8% | 18% | 28% |
2250 | 390 | 167 | 392 | 617 | 7% | 17% | 27% |
2275 | 389 | 163 | 391 | 618 | 7% | 17% | 27% |
2300 | 386 | 159 | 389 | 619 | 7% | 17% | 27% |
2325 | 384 | 155 | 387 | 620 | 7% | 17% | 27% |
2350 | 382 | 150 | 385 | 620 | 6% | 16% | 26% |
2375 | 379 | 145 | 383 | 620 | 6% | 16% | 26% |
2400 | 376 | 140 | 380 | 620 | 6% | 16% | 26% |
2425 | 373 | 134 | 377 | 619 | 6% | 16% | 26% |
2450 | 370 | 128 | 373 | 618 | 5% | 15% | 25% |
2475 | 367 | 122 | 370 | 617 | 5% | 15% | 25% |
2500 | 363 | 115 | 365 | 615 | 5% | 15% | 25% |
2525 | 360 | 108 | 360 | 613 | 4% | 14% | 24% |
2550 | 356 | 100 | 355 | 610 | 4% | 14% | 24% |
2575 | 352 | 91 | 349 | 606 | 4% | 14% | 24% |
2600 | 348 | 82 | 342 | 602 | 3% | 13% | 23% |
2625 | 344 | 72 | 334 | 597 | 3% | 13% | 23% |
2650 | 340 | 61 | 326 | 591 | 2% | 12% | 22% |
2675 | 336 | 49 | 316 | 584 | 2% | 12% | 22% |
2700 | 332 | 35 | 305 | 575 | 1% | 11% | 21% |
2725 | 327 | 20 | 293 | 565 | 1% | 11% | 21% |
2750 | 323 | 4 | 279 | 554 | 0% | 10% | 20% |
2775 | 318 | -14 | 264 | 541 | -0% | 10% | 20% |
2800 | 314 | -33 | 247 | 527 | -1% | 9% | 19% |
2825 | 309 | -55 | 227 | 510 | -2% | 8% | 18% |
2850 | 304 | -80 | 205 | 490 | -3% | 7% | 17% |
2875 | 299 | -107 | 181 | 468 | -4% | 6% | 16% |
2900 | 295 | -137 | 153 | 443 | -5% | 5% | 15% |
2925 | 290 | -170 | 122 | 415 | -6% | 4% | 14% |
2950 | 285 | -207 | 88 | 383 | -7% | 3% | 13% |
2975 | 280 | -249 | 49 | 346 | -8% | 2% | 12% |
3000 | 275 | -295 | 5 | 305 | -10% | 0% | 10% |
Exponent Factor = 1.123 For collapsing
1. Subject to the restriction in guideline “c)”
source : www.portlandoregon.gov