# 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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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'.

Vivian AlbanoFantastic article. Much obliged.

Noe MccayI really liked your article post.Really thank you! Will read on…

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.[…] Calgary Transit might send extra trains during events) it can move 9600 people/hour. A general theoretical maximum for saturated vehicle flow such as after a Flames game is 1900/hour/lane, which means even if cars […]

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?

Peak hour capacity is 55 seconds per vehicle? That metric makes no sense.

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!

I need to point out that over-all Im pretty taken with this web-site. Its very clear that you are aware of your subject matter and you are enthusiastic about it. I wish I possessed your ability to post. I have saved your blog and look ahead to additional posts.

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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.

fyi retail trips per day has increased 250%.

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.