December 9

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Numbers Every Traffic Engineer Should Know

By Mike Spack

December 9, 2011

Numbers Every Traffic Engineer Should Know, Traffic Corner Tuesday, Traffic Engineering Standards Traffic Facts
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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.

 

Mike Spack Bio

 

 

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  • 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'.

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

  • 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.
    Mike

  • 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.

  • 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.

  • “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,
    Mark

  • 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!

  • 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).

    Thanks

    Lisa

  • Hi Lisa – I don’t have any occupancy rates for the trip generation numbers. Mike

  • 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.

  • 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.

  • 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!

  • Mark,
    Any recommendations on preparing for PTOE? I guess memorizing those values won’t hurt lol. However, I’m not a big fan of paying big bucks for those ITE modules and am looking for a more feasible way to prepare.
    Any suggestions are welcome.

    Thanks

  • I wonder why the 1,900 vphpl theoretical capacity of uninterrupted vehicle flow per lane is important when every road has intersections that reduce that road’s capacity. I typically find that when analyzing congested intersections that the actual vehicle volume per lane on six lane roadways is closer to 550 to 1,000 vphpl than 1,900 vphpl depending on the crossroad through and turn volumes. If fact, sometimes the actual road capacity of three approach lanes is about the same as the uninterrupted flow one lane.

  • Well, he did say based on livability, not capacity. I think he is making a functional class distinction, not a giving a capacity threshold. Over 1000 vpd, it isn’t a local street anymore.

    Traffic noise gets intrusive, parents worry about their kids, etc., and you really need to treat it as a collector instead of a local street. Yes, you can get over 1000 veh/lane/hour on a two lane local road but should you?

  • Are there guidelines for how many lanes a freeway should have for given traffic volumes? This would be for a bypass type freeway. For example, how many lanes are recommended for a daily volume of say 55K and how many for 180K to 200K?

    Thanks.

  • Mike, can you tell me how many cars per hour is standard on a single family residential street? I am trying to figure whether intervention by the City is needed. Thanks, Blake

  • A residential street typically has less than a thousand cars a day. A reasonable rule of thumb is that the peak hour traffic is 10% of the daily traffic, so the peak hour on the residential street shouldn’t have much more than 100 vehicles per hour (combined going both directions). Of course they might have zero cars per hour in the middle of the night.

  • Very good information here…what is your general opinion of a proposed 280 lot subdivision that is “land locked” with only one entrance. With that entrance exiting onto another residential street that exits onto a four lane hwy with center turning lane. I’m thinking that a traffic signal will be required at the four lane hwy?

  • I have trouble believing your numbers are consistent. 55 seconds per vehicle is 65 vehicles per hour, meaning a 29:1 ratio between a lane with and without a traffic light. I would believe five or 10 to 1, but 29 to 1 strains credulity.

  • Sorry Ray – please be explicit with your question. I’m not following what you’re getting at. Mike

  • Kevin – the devil’s in the details. That’s a large enough development that a full Traffic Impact Study should be done to answer your question. Mike

  • Mike
    Have you looked at these numbers for a walkable mixed use community. Seems like they might reduce somewhat.

  • Hi Gary – These numbers are definitely Rules of Thumb. You are right to point out that context matters and a deeper level of analysis needs to be done once you’re working on a specific project. These numbers are meant to be a reasonable response when a traffic engineer is put on the spot with a question. Mike

  • Hi Tim – I don’t know of a specific limit on the number of lanes for a highway, but there is definitely a practical limit that would be developed through analyzing the specific highway segment. The limits would be mostly impacted by the traffic volume level and weaving associated with on and off ramps. Mike

  • Just curious, with one traffic light, 65 cars per hour is a possible figure? What happens on a country two lane road, with already over 500 houses, adding 1106 new homes with two traffic lights, each with right angle turns? Just how can they be evacuated rapidly? I have asked for the county traffic study. Deer in head lights..JEC

  • Definitely get the traffic study. Traffic will grow significantly and noticeably, but the road/signal will likely operate acceptably without needing upgrades.

  • I’ve heard it as a rule of thumb and double-checked the daily traffic volumes we collect locally. It holds in nearly all of the traffic count data sets I’ve seen. But, I don’t have a national source to cite for you.

  • Good morning Mike. For development planning purposes, what is the capacity rule of thumb for a two-way two lane driveway crossing. I’ve always considered at least one driveway crossing for every 500 trips being generated by the development. Thank you for your time

  • Sorry Brent, I don’t know of a rule of thumb. You should be doing a full traffic impact study for a development that generates more than a 100 trips in the peak hour. That will help you determine how many accesses are needed. By that logic, under a 100 peak hour trips should be ok with a single driveway.

  • Perhaps there is a typo – capacity of 1000 veh per day – maybe should be per 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

  • Mike, having just returned from our travels from Fenton, Michigan through Ohio, Kentucky (Cumberland Gap), through Eastern Tennessee, To (South Of Atlanta)Georgia, and the return trip through Alabama(Tuscaloosa), Western Tennessee(Nashville), Kentucky (Louisville), Indiana(Indianapolis & Fort Wayne), and back to Home (through Ann Arbor), I MUST ASK one thing:
    Has the US Interstate System surpassed it’s Maximum Vehicular Volume already?
    I ask this understanding:
    (1) the original concept for it was military in nature, and trying to travel from one point to another creates “choke points” in traffic flow (Cincinnati OH, Atlanta GA, Tuscaloosa AL, Louisville KY, and Indianapolis IN, these are the ones I have JUST PASSED through)
    (2) any “improvements” come with the requisite delay in smooth, TIMELY, flow of traffic.
    (3) the neglect Legislators from ALL states allowed over the last 40 Years, has brought about a crises in the Interstate system that was unable to be conceptualized, or planned for in 1957.
    I was born in 1954, so I have seen the initial improvement the Interstate system when I-75 came through our area in Michigan in about 1964, and when US-23 came through later. I observed the roads when my father drove and then when I was able to drive……
    Our infrastructure is in crisis, there is no viable secondary system other than Mass Transit that has it’s inherent weaknesses as the Interstate Highway system does.
    Which brings me to ask the question I asked above:
    Has the US Interstate System surpassed it’s Maximum Vehicular Volume already?
    If so, How can this be solved?
    Are you the correct Expert to direct my questions to?
    Or is there a different person in another field of expertise I should direct the questions to?
    No disrespect is intended, I spent 43 years in Patient Care, and I understand Expertise in specific areas (Neonatal, Pediatric, Adult and Geriatric, Ventilator, Oxygen, & Medication Administration), I just want to direct my questions to the correct area of expertise.
    It would be unfair to ask for answers from anyone not fluent in the specific knowledge set necessary to answer it fully.

  • Hi Stephen – Overall, I don’t believe the interstate system has exceeded its capacity as the majority of miles are outside of congested urban areas. There are certain segments within urban areas that are at capacity during portions of the day. Mike

  • Jay – you are right that the daily capacity of a two lane road 10x 1,000 vehicles per day, but we reference a capacity of 1,000 vehicles per day based on livability. This is a subjective line, but I don’t know of a city that is ok with a 10,000 ADT on a purely local residential road. We’re in the gray area of engineering vs planning. Mike

  • Is there a lower limit for two lane intersecting traffic at which a roundabout should not be considered or constructed in a four lane arterial street and signals have not historically existed at this locatioin?

  • Frank – no – there isn’t a lower bound of traffic for which a roundabout should not be installed (per the Manual on Uniform Traffic Control Devices – the national standard). Mike

  • What does peak hour capacity at signaled intersection of 55s/veh mean ?

    And secondly, is there a way to determine what should be the signal timing of an Arterial road intersection ?

  • I would say the “rule” is 8% to 12% and since 10% is in the middle … it makes sense for estimating.

  • Mike:

    I’m working on a construction project; a roundabout. Currently, it’s a four way STOP sign controlled intersection with NB and SB volumes quite negligible. For construction purposes the contractor wants to eliminate a single WB left-turn bay that services a UPS facility nearby. His plan is to have all WB traffic operating from a single lane. 24-hr counts are as follows: WBLT: 1304 WB thru: 9127 and EB thru: 8534.

    Is this WBLT value too high to eliminate that WB turn lane?

    Thank you,
    CFM

  • I feel like it would be good to include other modes of transport, so that you know when to switch. If my calculations show that I need an 8-lane road, that should probably make me think about a tram or metro.

  • Mike… good information btw
    Is there an information source that would explain controls for a direct connection of a residential street (collector) to a 4-lane divided highway via a round-a-bout?

  • Hi Denny – No. We have clear guidelines for stop sign and traffic signal control in the Manual on Uniform Traffic Control Devices, but the use of roundabout control is more subjective and left up to state and even local county/city guidelines. You’ll need to consult your local engineers. Mike

  • Hi Charles – you’ll want to run a capacity analysis or micro-simulation. My hunch is the roundabout would work without a WB Left Turn Lane at it, but you should confirm that based on peak hour turning movements against the capacity of the proposed roundabout. Mike

  • We have a developer wanting to build an apartment complex. They want to build 250+ apartments, my main concern is the main entrance and exit is a 2 lane road which is already used by 150 or so homes. The other exit leads to this road and another one that also had another 150 homes or using it. I am mostly of course worried about the density. How many units can they build, I am thinking closer to 100. We do not have any sidewalks in our neighborhood and people walk the streets all the time. Thanks

  • Hi Mike, thank you so much for this site, very helpful! I’m particularly interested in the apartments/condos stats and how that may have changed since 2018 with behaviour changes due to Covid and Amazon Prime/food deliveries etc. Do you know of any up to date studies relating to this?

    Thank you!

  • Hi Liz – I don’t have any updated trip generation data. Hopefully, this is something ITE is studying. Mike

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