New York State Department of TransportationJoseph H. Boardman, CommissionerDouglas A. Currey, P.E., Regional Director
(NYSDOT) is studying options for the reconstruction or replacement of theGowanus Expressway. Several project alternatives are under consideration.As part of this effort, NYSDOT is preparing a comprehensive EnvironmentalImpact Statement to help determine the best way to improve theexpressway. Traffic studies are a key element of this process.
These studies, nowunderway, will help plannersand engineers evaluate how well the GowanusExpressway and nearbyroadways are currentlyoperating and how trafficconditions might change inthe future. Using thisinformation, they can thenassess each projectalternative’s ability tohandle future traffic. Thedata are also used to predicteffects on air quality andnoise levels and to developstrategies to construct theproject in the leastdisruptive way to theadjacent communities.
PEDESTRIAN COUNTS are taken at intersections with espe-cially high pedestrian volumes, mainly along streets leading toschools, hospitals, and other community facilities, businesses,subway stations, and large apartment complexes. Traffic engineersuse this information to improve the safety of pedestrian crossingsby reducing the width of the roadway crossing, by making suretraffic signals allow enough time to cross the street safely, and byimproving lighting and signs.
TRAVEL TIME RUNS provide a statistically reliable measure ofaverage speeds along key roadways in the study area. Typically,survey crews drive along a stretch of roadway, traveling at theaverage speed of surrounding traffic. They note the time it takesto travel between points and the causes of delay, such as trafficsignals, construction, and double-parked cars.
ORIGIN AND DESTINATION (O/D) SURVEYS are used toobtain information on drivers’ travel patterns. Questionnaires askmotorists for information on where they began and ended theirtrips, how they normally travel (car, truck, transit, etc.), the road-ways they used, the time and purpose of their trip (going to work,shopping, etc.), and the number of occupants in their vehicle.This information is used to predict how travelers will react tochanged conditions on highways or local streets.
ACCIDENT ANALYSES document the accidents that occur inthe area. Each accident is categorized by location, type (e.g.,rear-end collision) and possible causes (following the car in fronttoo closely, weather, roadway conditions, low light, etc.). Accidentrates are then compared with statewide averages on comparableroads, and high accident locations are identified. This informationis used to design roadway improvements to reduce the likelihoodof accidents.
Hamilton Avenue underneath viaduct
ONDITIONS
Verrazano- Narrows Bridge
BrooklynBatteryTunnel
NewJersey
StatenIsland
UpperNew York
Bay
PRO
SPECT
SHORE PARKW
AY
GOW
ANU
SEX
PRES
SWAY
EXPR
ESSWAY
Brooklyn
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BROOKLYN-QUEENS
EXP
WY
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Entering the Gowanus Expressway fromthe Verrazano-Narrows Bridge
WHAT AREA IS STUDIED?The first step in a traffic study is to establish a TRAFFICSTUDY AREA, which includes all roadways and intersectionsthat could be affected by changes brought about by the project.The Gowanus Expressway project’s primary traffic study area (left)includes the expressway and intersecting highways (Shore Parkway,Prospect Expressway, Brooklyn-Queens Expressway), major avenuesand cross streets (such as Third and Hamilton Avenues and 65thStreet), and nearby local streets. The secondary area extendsacross the five boroughs and into New Jersey.
COLLECTING CURRENTINFORMATIONTraffic studies start with a comprehensive collection of the data needed to establish and analyze current, or BASELINE,traffic conditions. These include the roadway’s PHYSICALCHARACTERISTICS—how wide is the road, how many lanesdoes it have, are there any traffic signals, restrictions on parking?What is the posted speed limit? What is the road’s grade (slope)?These characteristics collectively determine a road’s ability tocarry traffic smoothly.
The next step is to survey present traffic volumes and conditions.The following studies are typical:
TRAFFIC COUNTS record the number and type of vehiclesusing a particular section of roadway. These samples are taken atdifferent times of the day, week, and year during representativetimes of the year, avoiding holidays, weekends, summer months,and unusual weather conditions. Counts are used to determinetraffic volumes and variations throughout the day, week, and year.
There are two major ways to count traffic: by machine andmanually. AUTOMATIC TRAFFIC RECORDERS (ATRS) usea rubber tube laid across the road to measure the number ofvehicles passing a particular stretch of roadway. ATRs continuouslycollect data over long periods of time, typically one to two weeks.
MANUAL TRAFFIC COUNTS, taken by survey crews, are usedto determine traffic volumes at local intersections.
CLASSIFICATION COUNTS are taken to measure the mix ofvehicles (cars, taxis, buses, motorcycles, and various types oftrucks) on a given roadway. This information is used to conductlevel of service studies and analyses of air and noise quality.
OCCUPANCY COUNTS are taken to measure the number ofpeople in cars on the expressway and major local streets. Thisinformation is important, since greater use of buses and car poolson a roadway can significantly increase its PEOPLE-HANDLINGCAPACITY without increasing the number of vehicles.
CURRENT C
PRIMARYTRAFFICSTUDYAREA
Gowanus interchanges with the BQEand Brooklyn-Battery Tunnel
HOVDuring the morning rush hours, a
Manhattan-bound Bus/HOV (high-occupancy vehicle) lane operates on the
Gowanus Expressway between the Verrazano-Narrows Bridge and the Brooklyn-Battery Tunnel,supplementing the three general-traffic lanes. Thelane is currently restricted to buses, cars, vans, andmotorcycles carrying two or more occupants and using
E-Z Pass. The Bus/HOV lane is only one of four Manhattan-bound lanes, but its combination of buses and car pools
carries over three-fourths of the highway’s total passengers.
HOW ARE TRAFFIC CONDITIONSEVALUATED?After the collection of traffic data is completed, traffic engineersuse a series of computer programs to evaluate how well highways,intersections and local streets are operating. These programsconsider the physical characteristics that determine the highwaysegment’s capacity. CAPACITY represents the maximum number ofvehicles that can travel on a roadway over a given period of time.
The software measures the ability of highways and local streets tocarry traffic by computing the LEVEL OF SERVICE (LOS) theyprovide. LOS values (from A to F) reflect the quality of traffic flow—A indicates free-flowing traffic conditions, while F describes stop-and-go driving conditions with lengthy delays.
The physical characteristics of the roadway itself (including, forexample, the number and width of lanes), the overall volume oftraffic, the mix of vehicles (trucks, cars, buses, etc.), and traffic
Level ofService
A
Level ofService
C
Level ofService
F
movements (vehicles getting on and off the expressway) all affectan expressway’s level of service. The programs calculate trafficDENSITY (how many vehicles are in a length of highway at agiven time), then use that information and the highway’s trafficpatterns to calculate level of service.
Essentially, the programs use the same factors that all driversknow cause delay: too many vehicles trying to get through, toomany vehicles switching lanes to get on or off the highway, toomany large trucks. Some or all of these conditions explain thetraffic delays that commonly occur during rush hours on urbanexpressways like the Gowanus.
The baseline studies create a comprehensive picture of conditionson the expressway and throughout the traffic study area. Withthis knowledge, traffic engineers and designers can developstrategies to improve the roadway’s traffic flow.
The next step is to develop tools to forecast future conditions.
Traffic volumes on the Gowanus areheaviest as the expressway passes overthe Gowanus Canal (below). Here, theexpressway carries vehicles headed toand from the Brooklyn-Battery Tunnel,the Brooklyn-Queens Expressway (BQE),the Prospect Expressway, and HamiltonAvenue. Traffic flows are furthercomplicated by the large number ofvehicles that must merge and maneuveracross lanes to make their connections.Drivers headed for the BQE must allmerge to the right to get to the twolanes leading to the BQE entrancelanes. In the same stretch of highway,drivers from the Prospect Expresswayheaded for the Brooklyn-Battery Tunnelare merging to the left. Thiscombination of high volumes andextensive merging commonly causescongestion and delays.
anual, Transportation Research Board, National Research Council, Washington D.C.
long-term projections of population, employment, and otherdemographic variables. The model combines these factors withland use projections—predictions of where new office space,shopping centers, and residential developments are likely to be built—which are based on data from local planning anddevelopment agencies, private real estate firms, and businessorganizations.
WHAT AREA DOES THEGOWANUS MODEL STUDY?The model’s primary focus is on the Gowanus Expressway andsurrounding areas. However, to understand travel patterns inthe Gowanus area, the model must look at all parts of theregion likely to generate trips that use the GowanusExpressway or nearby roads. This TRAVEL DEMAND AREAincludes all five boroughs of New York City and nearbycounties in New Jersey, which collectively produce the bulk oftrips made through the Gowanus area.
WHAT IS THE GOWANUS MODELUSED FOR?The model provides information on travel patterns that can beused to assess how well each alternative under considerationwould handle future traffic needs. The model can help predicthow travelers will deal with changed traffic conditions—willthey stay on the highway or shift to the local streets? willthey switch from cars to transit?
Using the model’s estimates, traffic engineers analyze thefuture levels of service on the area’s highways and local roads, employing the same computer programs they used toanalyze the baseline conditions. This knowledge allowsengineers and designers to compare and refine the variousalternatives to handle traffic more efficiently on theexpressway and local streets.
Ramp to the Gowanus from the Verrazano-Narrows Bridge
Ramps connecting the Gowanus andProspect Expressways
WHY LOOK AT FUTURE TRAFFICCONDITIONS? Highways need to last for decades. Because of this, plannersand engineers must project future traffic needs and use ofhighways. They must also assess how well each projectalternative—for example, reconstructing the present highwayor replacing it with a tunnel—will handle future traffic needs.
To answer these and other questions, NYSDOT is developing a comprehensive computer-based transportation model toproject future use of the Gowanus Expressway, other majorroads, and the area’s transit system. The technical name forthis type of model is TRAVEL DEMAND MODEL.
WHAT IS A TRAVEL DEMANDMODEL?A TRAVEL DEMAND MODEL is a collection of computerprograms that enables traffic planners to predict how travelpatterns will change in the future and to test alternative ways of handling these changed conditions. To do this, the modeldefines the factors that determine how many trips are made ina region, when the trips are made, what MODE is used (car,subway, bus, etc.), and what routes are taken.
Certain factors influence the number of trips in an area andhow they are made. These include demographic and economicfactors, such as number of residents and jobs, income levels,and auto ownership. The TRANSPORTATION NETWORKitself also influences how people make their trips: Whatsubways and buses are available and at what costs? How highare gasoline prices, tolls, and parking charges?
Modelers use statistics to quantify how each of these factorsaffects how people travel. By projecting these factors into thefuture (assuming, for example, more jobs, more residents, newshopping areas), they can estimate future travel patterns.
WHERE DO DATA ON FUTURECONDITIONS COME FROM?Travel demand models need large amounts of data—not only traffic data but also predictions of what and wheredevelopment will occur and changes in employment anddemographics (population, income levels). Transportation andplanning agencies in the region work together to establish
