Numbers Every Traffic Engineer Should Know

By Mike Spack, PE, PTOE

I originally posted this article several years ago. I attend a lot of meetings where I’m seen as the expert on traffic issues and I get asked questions related to basic standards and general practice. You can always respond that you don’t know the answer and you need to look it up, but you look better if you’re able to rattle off the numbers from memory. To that end, I have updated the list of questions and answers that every traffic engineer should consider memorizing.

About how much traffic will my development generate? (round numbers based on ITE Trip Generation Report, 10th Edition) (Corrections were made to these numbers as of as of Jan 25, 2018 at 10:00 pm)

  • Single Family Houses (per unit): 10 trips per day, 1 per peak hour
  • Apartments/Condos/Townhouses (per unit): 7 trips per day, 0.7 per peak hour
  • Office (per 1000 sq ft): 10 trips per day, 1.5 per peak hour
  • Retail (per 1000 sq ft): 38 trips per day, 4.2 per peak hour
  • Industrial (per 1000 sq ft): 5 trips per day, 0.9 per peak hour

Planning level daily capacity of a road (Round numbers based on Level of Service D/E thresholds in HCM 6th Edition)

  • 2 lane local street:  1,000 vehicles per day based on livability
  • 2 lane (w/ left turn lanes):  18,300 vehicles per day
  • 4 lane (w/ left turn lanes):  36,800 vehicles per day
  • 6 lane (w/ left turn lanes):  55,300 vehicles per day

Peak hour capacity of an intersection (Based on Level of Service D/E thresholds in HCM 6th Edition)

  • Stop sign controlled:  35 seconds/vehicle
  • Roundabout controlled: 35 seconds/vehicle
  • Traffic Signal controlled:  55 seconds/vehicle

Theoretical maximum saturation flow rate per lane (this will allow you to do quick calculations in your head to check reasonableness at big events)

  • 1,900 vehicles per hour per lane 

Threshold for when you need to add a second (dual) left turn lane at a signalized intersection?

  • 300 left turning vehicles from that leg of the intersection in the peak hour

Width of a commercial driveway (based on NCHRP Report 659)

  • One lane in only: 14 feet curb to curb
  • Two lane, bi-directional:  24 feet curb to curb
  • Three lane, one lane in with median than two lanes out: 40 feet curb to curb
  • Minimum industrial driveway:  26 feet curb to curb

Size of a footprint that a roundabout can take

  • Single lane diameter: 132 foot
  • Double lane diameter:  165 foot

Closeness between driveways and intersections (these are very rough rules of thumb — other regions are less stringent)

  • On a local street:  150 feet
  • On a collector street:  660 feet
  • On an arterial:  1,320 feet to 2,640 feet (with medians, right-in/right-outs can be 660 feet away)

Parking needed for functionality (these are suburban rules of thumb – urban conditions require less, but that’s very situational)

  • Multifamily Residential: 1 per bedroom
  • Retail: 4 per 1,000 SF
  • Restaurant: 15 per 1,000 SF (varies a lot)

Interested in learning more about the numbers every traffic engineer should know? Check out our Traffic Corner Tuesday webinar replay titled Numbers Every Traffic Engineer Should Know.


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19 thoughts on “Numbers Every Traffic Engineer Should Know

  1. Good stuff. I would add:
    Parking needed for functionality:
    Multifamily Residential: 1 per bedroom
    Retail: 4 per 1,000 SF
    Restaurant: 15 per 1,000 SF (varies a lot)
    Your peak hour capacities of intersections doesn’t make sense to me, you’re just showing the threshold for control delay going from D to E.
    Access control must be very strict in your area. Most places around here (DFW area) can have median openings at 400′ spacings, and on state roads usually about 650′. If signals are proposed at quarter mile spacings then it is “ample”, I don’t really get worried until it gets <800'.

  2. What is the hourly capacity per lane of a 2 lane urban arterial w/ a left turn lane? (assuming traffic signal optimization)

  3. Christopher,
    The hourly capacity of a 2 lane urban arterial depends on a lot of factors and comes down to more of an intersection analysis (as far as I know). The ideal capacity per lane is 1,900 passenger car equivalents per hour per lane. But a lot of deductions are taken for different circumstances and also assuming you rarely end up at capacity in both directions at the same time.
    Given the general estimating rule of the peak hour being 10% of the daily – you could benchmark the capacity at 1,600 vehicles as a rough starting point.

  4. Well done Mike. Good stuff.

    I think you need to take another look at the roundabout footprints. You list “Single lane diameter: 132 foot radius”. The phrasing is confusing, and the 132 feet would be the diameter, not the radius.

    Question for you – what is the basis of your number for daily capacity of a road. I get asked this one all the time and am always on the lookout for a good reference.

    • Mark – It’s not perfect, but Exhibit 16-14 Generalized Daily Service Volumes for Urban Street Facilities gives a range of capacities that I refer to regularly.

  5. On your daily capacity of a road you have 2 lane with CTL @ 16,000 ADT and 4 lane w/CTL @ 31,000. My question, can the 2 lane w/CTL exceed 16,000 (mid-block) if the intersections are 4 or more lanes w/CTL? I am assuming that your 2 lane / 16,000 ADT assumes that same 2 lane configuration at the (major) intersections.

  6. “Office (per 1000 sq ft): 1 trips per day, 1.5 per peak hour”

    I don’t see how that can be. Maybe it should be 10 trips per day, or 11 trips per day. How could it be 1.5 per peak hour, but only 1 per day?

    Otherwise, this is a wonderful post, with very useful numbers.

    Best wishes,

    • Hi Mark,
      Thank you for the catch. On further reviewing the numbers, there were several numbers that were off in the first section, “About how much traffic will my development generate?” I have updated those numbers this evening. Thanks for the heads up!

  7. Hi Mike,

    Do you have any information on the number of occupants assumed per HH in the trip generation rate? We are doing an analysis in a seasonal beach community where occupancy tends to be higher (along with day visitors).



  8. Mike-thanks for all these rules of thumbs for us rabble out there who are not on planning boards nor professional builders. Clarification: when you note trips you mean in and out of the driveways. In other words, 3 trips would mean the driveway would have a total of 3 cars that either in combination drove in or drove out of the project. BTW-is this national or depends on each state? I’m in NJ, unsure where you hail from. Our towns latest example BTW: we have a 298 rental residential unit with one drive way, 2 lanes each way onto a county road. Looking at your rule of thumb with .7 trips per unit per hour we would have 298 x .7=209 trips per hour divided by 60 min/hr=3.48 trips per minute. In our trffic expert testimony he said it generates 3 trips, 2 out 1 in for morning rush…so thats close to yur number which yours would be considered more conservative. Would you round yours out, then it would be considered 4 trips, right? Thanks for the metrics us residents can use for verification of testimony, and we understand its only rough.

  9. Mike, I notice you list the capacity of a two lane local street at 1000 vehicles per day. Thus cannot be correct, you must mean 10,000. If one house generates 10 trips per day, that means a two lane road could only serve 50 homes per side?

    • I had the same question and can only assume Mike meant 1000 VPH. It’d be nice to know if this rule can be tied to a specific table in the HCM too.

  10. I had a neighbor quote these trip generation numbers yesterday at a public committee meeting. He then went on to say that my traffic impact study was simply wrong because I had not used the above averages. While agree that these are all numbers we should be able to supply from memory, I think you should modify the trip generation section to indicate that these are the average rate and should not be applied for reviewing a study.

    On the plus side he said he googled trip generation and this was the first article he found!