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Traffic Control Need Calculator

This calculator is the primary step in determining the type of intersection control, but is only one element in an engineering study. Additional Items to consider and include in the engineering study are as follows:

  1. Controlling the direction that conflicts the most with established pedestrian crossing activity or school walking routes;
  2. Controlling the direction that has obscured vision, dips, or bumps that already require drivers to use lower operating speeds;
  3. Controlling the direction that has the best sight distance from a controlled position to observe conflicting traffic; and
  4. An intersection of a less important road with a main road where application of the normal right-of-way rule or yield signs would not be expected to provide reasonable compliance with the law;
  5. The need to control left-turn conflicts;
  6. The need to control vehicle/pedestrian conflicts near locations that generate high pedestrian volumes;
  7. Locations where a road user, after stopping, cannot see conflicting traffic and is not able to negotiate the intersection unless conflicting cross traffic is also required to stop; and
  8. An intersection of two residential neighborhood collector (through) streets of similar design and operating characteristics where multi-way stop control would improve traffic operational characteristics of the intersection.

This module shows what type of traffic control may be appropriate for an intersection based on a variety of inputs. The inputs can be used separately or in combination based on the amount of information available. This module is intended to be an approximation of appropriate traffic control, and is not a substitute for an engineering study.

  1. Based on available information, enter as much input information as is known. The calculator will work without information in all fields, however the results will be better if all information is known.
  2. Click the "Calculate" button at the bottom of the screen to determine the appropriate traffic control, based on the input information provided. This result will appear at the bottom of the module.
  3. To remove all entries and calculations, click the "Reset" button.
Possible Traffic Control NeedApproved
  

Intersection Sight Distance Calculator

This module presents many different options for calculating intersection sight distance depending on the intersection control and turning movement. Further help information will become available once a case has been selected.

  1. Select a case from the drop-down menu.
  2. Enter requested data for case selected.
  3. Click the "Calculate" button.
  4. The required minimum sight distance will be displayed in the white box.
  5. To remove all entries and calculations, click the "Reset" button.
Intersection Sight DistanceApproved
  

Intersection Sight Distance – No Control

This module calculates the required sight distance for vehicles approaching an intersection with no control on both approaches. The calculation is performed separately for each street. Note that if the grade or design speed varies for the major street, the calculation must be ran for each approach.

  1. Select the design speed from the first drop-down menu.
  2. Select the grade of the approaches.
  3. Click the “Calculate” button.
  4. The required minimum sight distance for the approaches will be displayed in the white boxes.
  5.  To remove all entries and calculations, click the “Reset” button.

This module is based on the AASHTO Greenbook Case A.

Note from AASHTO:
"If the sight distances given in [the table above] cannot be provided, consideration should be given to installing regulatory speed signing to reduce speeds or installing stop signs on one or more approaches."

Intersection Sight DistanceApproved
  

Intersection Sight Distance – Stop Control: Left-Turn from the Minor Road

This module calculates the required sight distance for vehicles turning left from a minor road with STOP control onto a major road. This sight distance is needed for motorists to see oncoming traffic from both directions in order to determine if they have enough time to complete their left turn maneuver.

  1. Select the Vehicle Type from the first drop-down menu.
    1. Passenger Cars are typical POV vehicles including pick-up trucks
    2. Single-Unit Trucks are typical non-articulated delivery trucks
    3. Combination Trucks are tractor-trailers
  2. Select the Design Speed from the second drop-down menu.
    1. If the design speed is not known, use the posted speed limit plus up to 10 additional mph.
  3. If the intersection approaches do not meet at a right angle (90°), enter the skew angle
    1. The skew angle must be less than or equal to 90°
  4. Select the Grade from the third drop-down menu.
    1. Level terrain is included in the -3% to +3% range
  5. Select the Number of Lanes to be crossed from the fourth drop-down menu
    1. This only counts the lanes being crossed, not the receiving lane
    2. For example, if the major roadway has 2 lanes in each direction, the turning vehicle will cross 2 lanes to complete the left-hand turn
    3. If the major roadway has a left-turn lane, include this as a lane
  6. If the major roadway has a median, enter the width in the final entry box
  7. Click the “Calculate” button.
  8. The required minimum intersection sight distance will be displayed in the white box.
  9.  To remove all entries and calculations, click the “Reset” button.

It may be necessary to verify that the sight distance is also sufficient for right-turns and crossing maneuvers.

This module is based on the AASHTO Greenbook Case B1.

Note from AASHTO:
"If the sight distance along the major road [calculated from the equations and adjustments] cannot be provided, then consideration should be given to installing regulatory speed signing on the major-road approaches."

Intersection Sight DistanceApproved
  

Intersection Sight Distance – Stop Control: Right-Turn from the Minor Road

This module calculates the required sight distance for vehicles turning right from a minor road with STOP control onto a major road. This sight distance is needed for motorists to see oncoming traffic from the left in order to determine if they have enough time to complete their right turn maneuver. It may be necessary to verify that the sight distance is also sufficient for left-turns and crossing maneuvers.

  1. Select the Vehicle Type from the first drop-down menu.
    1. Passenger Cars are typical POV vehicles including pick-up trucks
    2. Single-Unit Trucks are typical non-articulated delivery trucks
    3. Combination Trucks are tractor-trailers
  2. Select the Design Speed from the second drop-down menu.
    1. If the design speed is not known, use the posted speed limit plus up to 10 additional mph.
  3. If the intersection approaches do not meet at a right angle (90°), enter the skew angle
    1. The skew angle must be less than or equal to 90°
  4. Select the Grade from the third drop-down menu.
    1. Level terrain is included in the -3% to +3% range
  5. Select the Number of Lanes to be crossed from the fourth drop-down menu
    1. This only counts the lanes being crossed, not the receiving lane
    2. For example, if the major roadway has 2 lanes in each direction, the turning vehicle will cross 2 lanes to complete the left-hand turn
    3. If the major roadway has a left-turn lane, include this as a lane
  6. Click the “Calculate” button.
  7. The required minimum intersection sight distance will be displayed in the white box.
  8. To remove all entries and calculations, click the “Reset” button.

This module is based on the AASHTO Greenbook Case B2.

Note from AASHTO:
"If the sight distance along the major road [calculated from the equations and adjustments] cannot be provided, then consideration should be given to installing regulatory speed signing on the major-road approaches."

Intersection Sight DistanceApproved
  

Intersection Sight Distance – Stop Control: Crossing Maneuver from the Minor Road

This module calculates the required sight distance for vehicles on a minor road with STOP control crossing a major road. This sight distance is needed for motorists to see oncoming traffic from both directions in order to determine if they have enough time to complete their crossing maneuver. It may be necessary to verify that the sight distance is also sufficient for left-turns and right-turns.

  1. Select the Vehicle Type from the first drop-down menu.
    1. Passenger Cars are typical POV vehicles including pick-up trucks
    2. Single-Unit Trucks are typical non-articulated delivery trucks
    3. Combination Trucks are tractor-trailers
  2. Select the Design Speed from the second drop-down menu.
    1. If the design speed is not known, use the posted speed limit plus up to 10 additional mph.
  3. If the intersection approaches do not meet at a right angle (90°), enter the skew angle
    1. The skew angle must be less than or equal to 90°
  4. Select the Grade from the third drop-down menu.
    1. Level terrain is included in the -3% to +3% range
  5. Select the Number of Lanes to be crossed from the fourth drop-down menu
    1. Include all lanes in the roadway, including left-turn lanes
  6. If the major roadway has a median, enter the width in the final entry box
  7. Click the “Calculate” button.
  8. The required minimum intersection sight distance will be displayed in the white box.
  9. To remove all entries and calculations, click the “Reset” button.

This module is based on the AASHTO Greenbook Case B3.

Note from AASHTO:
"In most cases, the departure sight triangles for left and right turns onto the major road, as described for Cases B1 and B2, will also provide more than adequate sight distance for minor-road vehicles to cross the major road. However, in the following situations, it is advisable to check the availability of sight distance for crossing maneuvers:

  • where left or right turns or both are not permitted from a particular approach and the crossing maneuver is the only legal maneuver;
  • where the crossing vehicle would cross the equivalent width of more than six lanes [72-ft]; or
  • where substantial volumes of heavy vehicles cross the highway and steep grades that might slow the vehicle while its back portion is still in the intersection are present on the departure roadway on the far side of the intersection."

For divided highways, if the median is not wide enough to store a vehicle, the fL factor includes all the lanes in the cross section. If the median is wide enough to store a vehicle, the sight distance can be considered twice: first for crossing the near side of the roadway; second for crossing the far side of the roadway beyond the median.

Intersection Sight DistanceApproved
  

Intersection Sight Distance – Yield Control: Left- or Right-Turn from the Minor Road

This module calculates the required sight distance for vehicles turning left or right from a minor road with YIELD control onto a major road. This sight distance is needed for motorists to see oncoming traffic in order to determine if they have enough time to complete their crossing maneuver.

  1. Select the Vehicle Type from the first drop-down menu.
    1. Passenger Cars are typical POV vehicles including pick-up trucks
    2. Single-Unit Trucks are typical non-articulated delivery trucks
    3. Combination Trucks are tractor-trailers
  2. Select the Design Speed from the second drop-down menu.
    1. If the design speed is not known, use the posted speed limit plus up to 10 additional mph.
  3. If the intersection approaches do not meet at a right angle (90°), enter the skew angle
    1. The skew angle must be less than or equal to 90°
  4. Select the Number of Lanes to be crossed from the fourth drop-down menu
    1. For left-turns, the number of lanes to cross does not include the receiving lane
    2. For right-turns, the number of lanes to cross is zero (0).
  5. If the major roadway has a median, enter the width in the final entry box
    1. This entry is not applicable for right-turns.
  6. Click the “Calculate” button.
  7. The required minimum intersection sight distance will be displayed in the white box.
  8. To remove all entries and calculations, click the “Reset” button.

This module is based on the AASHTO Greenbook Case C2.

Intersection Sight DistanceApproved
  

Intersection Sight Distance – Yield Control: Crossing Maneuver from the Minor Road

This module calculates the required sight distance for vehicles on a minor road with YIELD control crossing over a major road. This sight distance is needed for motorists to see oncoming traffic from both directions in order to determine if they have enough time to complete their crossing maneuver.

  1. Select the Vehicle Type from the first drop-down menu.
    1. Passenger Cars are typical POV vehicles including pick-up trucks
    2. Single-Unit Trucks are typical non-articulated delivery trucks
    3. Combination Trucks are tractor-trailers
  2. Select the Design Speed for the Major Road from the second drop-down menu.
    1. If the design speed is not known, use the posted speed limit plus up to 10 additional mph.
  3. Select the Design Speed for the Minor Road from the third drop-down menu.
    1. If the design speed is not known, use the posted speed limit plus up to 10 additional mph.
  4. If the intersection approaches do not meet at a right angle (90°), enter the skew angle
    1. The skew angle must be less than or equal to 90°
  5. Select the Grade from the third drop-down menu.
    1. Level terrain is included in the -3% to +3% range
  6. Enter the width of the major road that is being crossed in the final entry box.
    1. Assume each lane is 12-ft wide unless specific measurements are available
    2. Median widths, if present, vary by location
  7. Click the “Calculate” button.
  8. The required minimum intersection sight distance will be displayed in the white box.
  9.  To remove all entries and calculations, click the “Reset” button.

This module is based on the AASHTO Greenbook Case C1.

Note from AASHTO:
"If the sight distance along the major road [calculated from the equations and adjustments] cannot be provided, then consideration should be given to installing regulatory speed signing on the major-road approaches."

Intersection Sight DistanceApproved
  

Intersection Sight Distance – Traffic Signal Control

This module calculates the required sight distance for vehicles at a signalized intersection. The operation of the signal and the prohibition of right-turns-on-red affect the requirement for sight distance.

  1. Toggle ‘Yes’ or ‘No’ for whether the signal will operate on flashing during off-peak hours.
    1. If ‘Yes’, sight distance must be calculated.
    2. If ‘No’, sight distance does not need to be calculated since vehicle entrance to the intersection is controlled by the signal phasing. Refer to Step 4.
  2. Toggle ‘Yes’ or ‘No’ for whether right-turns-on-red are permitted.
    1. If ‘Yes’, sight distance must be calculated.
    2. If ‘No’, sight distance does not need to be calculated since vehicle entrance to the intersection is control by the signal phasing. Refer to Step 4.
  3. If ‘Yes’ is selected for either or both of the toggles, additional entry boxes will appear.
    1. Select the Vehicle Type from the first drop-down menu.
      1. Passenger Cars are typical POV vehicles including pick-up trucks
      2. Single-Unit Trucks are typical non-articulated delivery trucks
      3. Combination Trucks are tractor-trailers
    2. Select the Design Speed from the second drop-down menu.
      1. If the design speed is not known, use the posted speed limit plus up to 10 additional mph.
    3. If the intersection approaches do not meet at a right angle (90°), enter the skew angle
      1. The skew angle must be less than or equal to 90°
    4. Select the Grade from the third drop-down menu.
      1. Level terrain is included in the -3% to +3% range
    5. Select the Number of Lanes to be crossed from the fourth drop-down menu
      1. Include all lanes in the roadway, including left-turn lanes
    6. If the major roadway has a median, enter the width in the final entry box
    7. Click the “Calculate” button.
    8. The required minimum intersection sight distance will be displayed in the white box(es).
    9. To remove all entries and calculations, click the “Reset” button.
  4. If ‘No’ is selected for both of the toggles, a purple-shaded box will appear explaining that the sight distance requirement is that the first vehicle on an approach can see the first vehicle on each of the other approaches.  There is no actual calculation associated with this case.

This module is based on the AASHTO Greenbook Case D.

Intersection Sight DistanceApproved
  

Intersection Sight Distance – All-Way Stop Control

This module explains the sight distance requirement for an intersection with STOP control on all approaches.  There is no actual calculation associated with this module.

This module is based on the AASHTO Greenbook Case E.

Intersection Sight DistanceApproved
  

Intersection Sight Distance – Left-Turns from Major Road

This module calculates the required sight distance for vehicles making a left-turn from a major road onto a minor road. This sight distance is needed for motorists to see oncoming traffic on the major road in order to determine if they have enough time to complete their crossing maneuver.

  1. Select the Vehicle Type from the first drop-down menu.
    1. Passenger Cars are typical POV vehicles including pick-up trucks
    2. Single-Unit Trucks are typical non-articulated delivery trucks
    3. Combination Trucks are tractor-trailers
  2. Select the Design Speed from the second drop-down menu.
    1. If the design speed is not known, use the posted speed limit plus up to 10 additional mph.
  3. If the intersection approaches do not meet at a right angle (90°), enter the skew angle
    1. The skew angle must be less than or equal to 90°
  4. Select the Number of Lanes to be crossed from the fourth drop-down menu
  5. Click the “Calculate” button.
  6. The required minimum intersection sight distance will be displayed in the white box(es).
  7. To remove all entries and calculations, click the “Reset” button.

This module is based on the AASHTO Greenbook Case F.

Intersection Sight DistanceApproved
  

Storage Length Calculator

This module calculates the required length to accommodate the queue of vehicles waiting to make a left-turn.

  1. Select the approximate number of left-turn movements in the peak hour from the drop-down menu.
  2. Toggle the appropriate intersection control: signalized or unsignalized.
  3. If the intersection is signalized, choose the signal length (either by seconds or phasing)
  4. Press the “Calculate” button.
  5. The required storage length will be displayed in the white box on the graphic.
  6. To remove all entries and calculations, click the “Reset” button.
Storage LengthApproved
  

Approach Taper Length Calculator

This module calculates the length of lane shift tapering required to add a left-turn lane at an intersection approach. The shifting taper is needed so that the redirection of traffic around the left-turn lane is not too abrupt. The length of the taper depends on the speed of traffic on the roadway and the width to be shifted.

All lanes are assumed to be 12-ft.

  1. Select the design speed of the roadway from the first drop-down menu
    1. If the design speed is not known, use the posted speed limit plus up to 10 additional mph.
  2. Select the number of left-turn lanes that traffic must shift around
  3. Press the “Calculate” button.
  4. The required approach taper length will be displayed in the white box on the graphic.
  5. To remove all entries and calculations, click the “Reset” button.
Approach Taper LengthApproved
  

Bay Taper Length Calculator

This module calculates the length of taper required to open up a left-turn lane. The bay taper is needed so that the redirection of traffic around the left-turn lane is not too abrupt. The length of the taper depends on the speed of traffic on the roadway and the width to be shifted.

All lanes are assumed to be 12-ft.

  1. Select the design speed of the roadway from the first drop-down menu
    1. If the design speed is not known, use the posted speed limit plus up to 10 additional mph.
  2. Select the number of left-turn lanes that traffic must shift around
  3. Press the “Calculate” button.
  4. The required bay taper length will be displayed in the white box on the graphic.
  5. To remove all entries and calculations, click the “Reset” button.
Bay Taper LengthApproved
  

Stopping Sight Distance Calculator

This module calculates the required minimum stopping sight distance based on the design speed and grade of a roadway. Stopping sight distance refers to the distance a typical driver will use between recognizing the need to stop and coming to a complete stop.

  1. Select the design speed of the roadway from the first drop-down menu.
    1. If the design speed is not known, use the posted speed limit plus up to 10 additional mph.
  2. Select the approximate grade from the second drop-down menu.
    1. Positive grades are uphill; negative grades are downhill.
  3. Press the “Calculate” button.
  4. The required stopping sight distance will be displayed in the white box.
  5. To remove all entries and calculations, click the “Reset” button.
Stopping Sight DistanceApproved