Skip to Main Content View Text-Only

Portland Bureau of Transportation

We keep Portland moving

Phone: 503-823-5185

Fax: 503-823-7576

1120 SW Fifth Ave, Suite 800, Portland, OR 97204

More Contact Info

Impact Threshold Curve

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; and
 
c) 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.
 
c) The resulting traffic volume on any local service street should not exceed 3,000 vpd.
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)"